Broken redirects is certainly a major SEO mistake. They range from unnecessary hops from one URL to another to another, which slows the final page load down but doesn’t cause a major problem, right through to infinite redirect loops which completely crash the user experience and can get sites totally de-indexed as if banned.
Duplicate content is definitely a common one 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 duplicated vs unique content, and how many words in a chain are repeated, etc.
Another very common technical SEO mistake is broken links, ie they just 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 500 (server error), this is common on messy large ecommerce sites and is terrible for SEO.
‘403 – forbidden’ is another common one which denotes blocked content by robots.txt.
Dangerous spam signals like hidden text, especially hidden links to external sites, are also about as bad an error as one can get but they are still very common. Small text external links and low contrast / hard to read links are almost as bad and are also very common.
Failure to add pages to the menu
Failure to add pages to the menu or other internal linking structure is also a prevalent issue which can cause solid content not to rank at all. There is a similar thing observed where the homepage is a splash page with a minimal design of its own, then there’s a large site behind it but no link from the homepage to the inner site.
Old site pages still available when new page is live
Having old sites’ pages still there when new pages are live is also a common problem. 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.
Here’s a huge one: non responsive layout (not a mobile friendly design). Google indexes sites based on how the mobile version loads and performs, so even if you have superb content which loads correctly on a desktop, you may still lose ranking if your site loads poorly on mobile devices.
SEO time management
Another key technical seo error is spending too much time on trivial technical issues when there is more rewarding work to be done like creating new content or promoting it.
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 said. SEO is really a complex messy industry that few people take the time to organise to such a level as this.