Google will find out sooner rather than later if you use any Bad SEO tactics that fall outside the ‘Google Webmaster Guidelines' https://support.google.com/webmasters/answer/35769?hl=en and penalize you harshly by retiring your site to the 'never to be found', 'ranking on the 100th page', 'low quality website' realm.
The Bad Boys of SEO
1. Buying Backlinks
Backlinks occur naturally from website to website through partnerships within businesses, the sharing of relevant information and links via social media channels and associated businesses, which are all good for SEO rankings.
However, there are a number of companies selling backlinks which offer an easy option to initially gain results but ultimately will kill your rankings. As a rule, buying or gathering unnatural links is a huge no no – don't be fooled.
Take a little time to build up relevant and interesting articles and blogs, linked and shared with reputable and associated sources to enable natural, long standing and recognized links and keep those search engines on your side.
2. Keyword Stuffing
Repeating keywords over and over again in your content, colloquially known as keyword stuffing is now considered bad SEO practice. Gone are the days of over using one keyword to stimulate page view results, you now have to be a little more considerate. Firstly, it is very boring for users to read keyword stuffed content and also makes you seem a bit silly and secondly, the search engines will know, if you repeat a keyword unnaturally over and over, that you are trying to cheat their algorithm’s. Naughty, naughty! Which also makes for a very angry Search Engine.
Instead, try to use lots of different relevant phrasing that would naturally occur within the subject matter of your content and use targeted keywords clearly in your URL's page titles and descriptions.
3. Duplicate Content, duplicate content, duplicate content …
There are 2 main examples of duplicate content.
a. Reusing your own content
You may be copying text from one page of your site to use on another page as it is covering the same topic. This stops the search engines recognising the individual page as it has already found the text before.
It is much more beneficial to write a unique text for each page, even if you are covering the same area – it will be more interesting for your users and the search engines will find each individual page more easily.
b. Content Plagiarism
We all do it – we read an article or content already out there on the web and think that's great! There is no harm in being inspired by ideas, blogs and articles. Using an idea and rewriting content in your own style and adding in your own research and take on things is how content is formed. But and this is a big BUT, copying and pasting paragraphs of text, word for word, without editing or changing any of the content, is plagiarism and wrong. And the search engines know it is wrong too!
4. Forgetting Your Mobile Users
It is amazing how many sites are still not mobile responsive. Back in April 2015 , Google Search expanded its use of mobile-friendliness as a ranking signal. This change had a significant impact in Google Search results for any websites that are not responsive or mobile user friendly site.
Here are some pointers to avoid when working on your mobile friendly site design.
a. Don't override user experience for looking good
It is very easy to mobile format an existing design so it looks lovely on a mobile or tablet, but has little or no useful functionality left after condensing the site to look good.
Always keep the mobile user in mind and include all the information and functionality of the desktop version on mobile version too by integrating mobile usability into the design.
b. Don't be tempted to have a separate mobile site url
Responsive web design is Google’s recommended configuration, so it is in your rankings best interest to use the same domain and Url for your mobile site, rather than using different domains, urls or subcategories for your mobile site.
c. Don't go it alone!
Look for inspiration and look at how your competitors have used responsive web design to improve mobile site usablility. Do your research into best practice and layout by learning from others who have already pathed the way with successful mobile responsive design.
Other tools that you can utilize and gain ideas from are The Mobile Playbook http://www.themobileplaybook.com/uk/#/cover and Google Multi-Screen Success Stories https://support.google.com/adsense/answer/6053464?hl=en-GB
5. Ignoring Google Analytics
Google offers a very important tool in the form of Google Analytics https://analytics.google.com/ , reporting on all aspects of your websites performance. And it is free! Not utilizing this information is an oversight that could ultimately damage your ability to rank well in google search.
It will change the way you view your own website and give you the data required to make important changes to enhance your website performance for the future.
Final thought - Don't be lazy!
Good SEO is about taking the time to get it right!
Create good keyword rich content, gain natural links by sharing that unique and interesting content through the appropriate channels.
Make sure your site is mobile friendly for users and ranking purposes, basically do your research. Always analyse your websites performance regularly to fully understand the impact of your input and changes, so you can continually improve your SEO rankings now and in the future.