Some blogging tools and CMS platforms make it all too easy to create duplicate metadata by accident. While metadata isn’t a cause for a penalty on its own, it can be a sign of a duplicate content issue on your site. In any case, it’s undesirable; try to deal with it.
Aside from the obvious 404 error, there are a range of others that Google really hates to see. 302 (temporarily moved) isn’t ideal; if you really must redirect something, use 301. Also, if you see any 500 errors, deal with the root cause as soon as you can. Find invisible errors with this WebConfs HTTP Header Check tool.
If you’ve received a penalty on your site, using a 301 redirect could transfer the penalty to a new location. What’s more, the penalty could linger if you remove the redirect later. To be safe, don’t do it.
When linking to other websites, keep it natural. A high quantity of links is a sign that you’re swapping links with people for the sake of mutual SEO benefit.
The controversy around advertorial content was perhaps the most well known of the pre Penguin 2 debates. An advertorial is basically a page of content riddled with paid links, and often these pages were being used for aggressive manipulation of search results. The most famous example was Interflora
Google doesn’t like to see too much of a good thing. An over optimization penalty usually means you’ve gone a step too far in your bid to obsessively out SEO everyone else in your industry. Cool it and publish some natural content before your rank suffers.
Businesses sometimes try to use multiple landing pages in order to improve their position in SERPs. Some companies also try to improve their position by creating lots of one page websites optimized for a single keyword, then funneling users through to another site. Google considers this kind of thing to be bad practice.
Never associate yourself with a website that is doing something ethically or legally dubious. Hacking, porn and malware ridden sites should be avoided. Also, try to remove links to other sites that have been penalized in the past, assuming you know about it.
We’ve all used forums awash with signature links. Sometimes there are so many, it can be hard to locate the actual posts. If you add a forum link, use good, natural linking techniques and consider making it a nofollow too.
The robots.txt file should be used to tell search engines how to deal with your site. While there are legitimate reasons for excluding pages from robots.txt, do it sparingly: excessive blocking could be the cause of your penalty.