Broken redirects
Having broken redirects is certainly a common SEO problem. They range from unnecessary hops from one URL to another and another, which slows the final page load down but doesn’t cause a major problem, right through to infinite redirect loops which crash the user experience and can get sites completely de-indexed as if they were penalised in the worst way until the issue is resolved.
Duplicate content
Duplicate content is definitely a common issue, and while its technical aspect is small, it is still something that people often do not think of until they know that Google penalises sites for it. It can get quite technical when considering what percentage of each page is boilerplate/duplicative vs unique content, and how many words in a string are repeated, etc.
Broken links
Another very common technical SEO mistake is broken links, ie, where they simply point to a non-existent location that may have once existed or may be a typo. Redirects often then kick in to send people who land on error pages to more useful pages, and this can get messy. Aside from broken links, which return Error 404 (Page Not Found) unless the server has bigger issues or redirects have kicked in to tackle the error, there are other errors like Error 500 (miscellaneous server error) – this is common on messy large ecommerce sites and is terrible for SEO. Error 403 (Forbidden) is another common one which denotes blocked content by the server for the current user (often in error, but potentially intentionally).
Spam signals
Dangerous spam signals like hidden text, especially hidden links to external sites, are also about as bad a mistake as one can get but they are still very common. Small text, low-contrast and other hard-to-read external links are almost as bad as fully hidden external links and are also very common.
Forgetting to link to new content
After creating new content, failure to link to it from established, well-crawled places on your site such as the main menu or any other internal linking structure with a root from the homepage is also a prevalent issue which can cause good content to not rank at all because search engines simple never know about it. XML Sitemaps are another means via which search engines can discover your content, but nothing beats the good old HTML link from well-crawled pages as this transfers link juice, and content relevance with that juice, to help your new pages rank especially well for key terms and topics that are found on the pages you linked from – especially the terms in the anchor text of the link itself, and then after that the directly surrounding text, etc.
Old (redundant) pages are still available when the new equivalent pages are live
Having a site’s old pages still there when new pages are live is a common problem. Not a big deal, just a missed opportunity to consolidate (a) traffic, and (b) link equity to boost the ranking power of the new pages. Old URLs should be dealt with, usually by redirection to the new equivalent URLs, rather than being left stale because search engines still send traffic to those old pages. Without setting up redirects, your old pages are likely to continue to get traffic because they’re established & trusted by Google, and if the content is out-of-date then that’s a bad impression to your users which is obviously not good but also causes them to bounce back to search results more readily which in turn tells Google your users were unimpressed and then your ranks will be slide down mainly for those disappointing redundant pages but this has a slight knock-on effect on the ranks of your entire site.
Non-responsive layout
Here’s a huge one: non responsive layout (not a mobile friendly design). Google ranks sites on mobile devices based on how the mobile version of the site loads & performs, so even if you have superb content which loads fast and looks great on a desktop/laptop computer, you may still lose rankings on mobile search if your site loads poorly on mobile devices, and due to the popular nature of link sharing, device switching and return visits, Google’s poor impression of your non widely accessible site will even somewhat lower its rankings across all devices (even the ones your site is fine on).
Mistaken priorities — time & resource wasted
A common strategic SEO error is investing too much time & resource on trivial technical tweaks, formalities & experiments just to look pretty for Google, when there is certainly highly rewarding work to be done elsewhere. Even if your site is technically in good shape, there’s still a high return to be gained from activities like creating new content, streamlining existing content, developing tools and utility value, improving user experience and conversion rate, and proactively promoting the site in various ways.
Missing keywords
We can’t miss the classic: not getting the relevant keywords on the page. The main mistake in SEO is having no site, or no crawlable site; next is having no keywords on pages.
This kind of top level ‘main mistake, next main mistake’ is rarely mentioned. SEO is really a complex messy industry that few people take the time to organise to such a level as this.