1. Ignoring Page Speed
  2. Irregular Site Audit
  3. Not Updated for Google Algorithm 
  4. Overlooking Basic Search Engine Optimization(SEO)
  5. Not Republish Outdated Content
  6. Not Optimizing Images
  7. Improper Keywords Usage
  8. Not Optimizing Heading 
  9. Making wrong Content Strategy
  10. Not Updating Old Content
  11. Wrong CTAs Placement
  12. Not Checking GSC Performance
  13. Poor Landing Page Design
  14. Ignoring EAT Factors
  15. Poor Navigation
  16. Not Checking Indexability and Crawlability
  17. No Schema Optimization
  18. No Link to Quality Websites
  19. Ignoring Mobile Friendliness and AMP
  20. Bad UI/UX
  21. Not Implementing Proper Sitemap and Robots
  22. Wrong Category and Tag usage
  23. No Social Presence
  24. Guest Posting on Spammy Websites 
  25. Not Filtering Spammy Backlinks
Let's check out each of these mistakes in detail and ways to refrain from them.

1. Ignoring Page Speed

It is the most important factor from the SEO Perspective. Many business owners ignore this. According to stats, 93% of the people will bounce from your website if your web page loads slow.
What to do if your website speed is slow:

2. Irregular Site Audit

Check the technical issues fixes of your website. Google loves websites that are error-free.
Having lots of issues on your website can damage your visitors' experience and can harm your website. Audit your website with technical SEO fixes on a weekly or monthly basis.
What are the basic errors to check :

3. Not Updated for Google Core Algorithm

Google almost changes its algorithms more than 500 times a year. These changes are very small and do not impact many results. But in a year, Google rolls out its Core Update of Algorithms twice or thrice.  These core algorithms update impact many websites and in many ways. So, it’s really important to keep an eye on Google Algorithms when they are rolled out.
You can use the following tools to check the Google Updates- SEMrush Sensor, Algoroo, Moz Cast, etc.
Some major Google Algorithm are: 

4. Overlooking basic SEO

Many websites ignore the basic HTML issues of  SEO and meta tags like title, description, etc.
Although these are basics of SEO but play a major role to increase your ranking on search engines. 
What to consider in basic SEO:

5. Not Republishing Outdated Content

Often, many bloggers and content writers don't bother to update the older content published on the website. Republishing old content again and again with the latest trend for the topic can give a boost to the ranking of the older blog.
What to do of the content which are outdated or obsolete:

6. Not Optimizing Images

Images are the decorative and attractive part which make your blogs less boring and help visitors’ to keep engaging with the article till the end. 
Images help visitors to understand more about the product and services.
Source: Bluecorona
Things to consider when using images:

7. Improper Keyword Usage

Using proper keywords in your content or on any product page will let the user know what exactly the content is about.
The words and phrases help in knowing the user about the blogs and give an overview of the search engines.
Keywords Research is also important because they improve your search engine rankings. Ranking high on search engines needs keyword research as a top priority for an SEO campaign.
In fact, we can say that without proper keyword research, implementing SEO is impossible.
Source: Backlinko
Pro-tips to use keywords properly:

8. Not Optimizing Headings 

Heading optimization leads to the proper text structure of the content. It also improves the accessibility and enhances SEO of the page. Heading also improves the readability of the page.
Tips to optimize the headings:

9. Framing Wrong Content Strategy

Planning a good content strategy with good topic scheduling can take your website to a higher level.
We all know that content is king. Content is the major part for which SEO is performed. Content is used everywhere on websites. Be it a landing page, blogs, articles, images, etc.
Some strategies to sum up while making content:

10. Not Updating Old Content

Republishing your old content is a must if it's outdated. According to stats, the older your content will be, the less chances to rank and visible over SERPs.
Google uses fresh content to determine your website rankings. To maintain yourself on top, you need to update your content consistently which boosts freshness and SEO.
If you are not interested in updating the old content and think it’s outdated or obsolete, you can remove it from your website.
Source: Neil Patel
What are the benefits to keep your content updated?

11. Wrong CTAs placement

CTAs or Call to Actions may not be a direct ranking factor but it can surely hurt your website and the user experience.
Having a good placement of CTAs can increase your sales and conversion by 200%. Making good and attractive CTAs can bring you leads.
Some tips to make a good CTAs:

12. Not Checking GSC Performance

It’s necessary to check the overall performance of the website via the free tool Google Search Console (GSC). Many business owners and websites don't have GSC or don't check the performance report of blogs or content. 
Google Search Console is a free tool and in SEO, it helps us to tell about the performance on the basis of Top Pages, Search Queries, Country, and Devices.
Top features to check on GSC are:

13. Poor Landing Page Design

Landing pages drive visitors through all the important links and sections where you want them to click and visit. You need to put important elements on your landing pages using which visitors can easily navigate throughout the website.
Some best practices to improve landing pages:

14. Ignoring EAT factors

EAT in SEO or Expertise, Authoritativeness, and Trustworthiness factors are the basic and important guidelines which Google provides for all websites.
It consists of concepts like Page Quality, YMYL (Your Money, Your Life), Need Met, and many other factors that should be checked by all websites to increase its effectiveness. It tells about the search quality of the pages existing on your website.
Some important criteria to check in EAT:

15. Poor Navigation

Navigation is crucial and must be optimized by every website. It can result in the best success in terms of conversions or can decline your website to failure and no sale.

It also impacts the user experience as well as the friendliness of the website.
Some major points which you should note:

16. Not Checking Indexability and Crawlability

Google bots check the pages on the website and then index and crawl your website. So it’s very important to check your website that it must not create any issues in crawling.
A checklist to review:

17. No Schema Optimization

Schema markup helps in letting Google know the proper structure of the website and boosts your SEO. It helps in showing featured snippets over search engines to help visitors get the knowledge quickly and accurately.
Some important schema markup are as follows: 

18. No Links to Quality Websites

Quality links to your website and on the website, both are important. The more easily link juice will pass, the more your website is read by Google in an efficient way. Make sure to add good external links on your website and try to acquire backlinks from authoritative websites.
Source: Neil Patel
Some points to recheck:

10. Ignoring Mobile Friendliness and AMP

Making you page mobile-friendly and implementing Accelerated Mobile Pages(AMP) is not that tiresome. But most of the websites take it lightly.
It helps the visitors to check your website and go through your pages easily. Google also considers AMP and mobile friendliness as a top priority while ranking websites.
Source: MakeWebBetter SEO tool
Tips for AMP and Mobile SEO: 

20. Bad UI/UX

Poor UI/ UX can result in the failure of your website. Whenever any visitor comes on your website, the first thing they feel and observe is the UI/ UX. It should be attractive and easy to navigate so that they can reach other pages without any difficulty.
Some tips for UI/UX:

21. Not Implementing Proper Sitemap and Robots

Sitemaps.xml helps the Google bot to review and go through the website pages linking to each other easily. It contains the list of pages according to their category and subcategory
Robots.txt tells which page should be crawled and which are not to be crawled. If you want any page not to crawl by google you can add them on the robot.txt file.
Some points to review:

22. Wrong Category and Tag Usage

Categories and tags help to boost SEO and help Google to understand your pages in a proper manner. 
WordPress Plugins like Yoast have an inbuilt feature to set the categories and tags in a user-friendly manner for Google as well as for visitors.
Some points to take care while using tags and categories:

23. No Social Presence

Nowadays, social media plays an important role for websites. If you don't have social media links on your website, then you must start doing it today. Social platforms boost your visibility and give positive signals to the search engines. Make a social media checklist where you can prepare the list of tasks you need to do on particular social media platforms.
Following are the channels with different benefits in terms of SEO:

24. Guest Posting on Spammy Websites 

Many SEO or website owners believe that doing guest posts will benefit your business and gain authority. It's really true but doing them in the wrong way can hurt your rankings too.
Some suggestions for guest posts:

25. Not Filtering Spammy Backlinks

Filtering backlinks is one of the most important SEO tasks which people ignore. But to be serious, you should check and audit your website’s backlinks on a weekly or monthly basis. It’ll help you to protect your website from spammy websites that link back to your domain anonymously.
Here are some todos:

Conclusion

Making a good and SEO friendly website needs consistent checks and audits. You need to check all points, ranging from technical to basic SEO. 
Performing SEO in an incorrect way can result in a bad health score of your website and hence will impact the overall performance of the website; not only in terms of SEO but also in terms of conversions and sales with loss in revenue. 
Take all precautionary measures and actions on your website and remain beloved in the eyes of Google and other search engines.