I used to provide consulting services to small companies and site owners. I became much more focused on web development, but SEO experience has been helpful in a number of ways since.

The biggest case I can remember is a former employer who ran a niche search engine. They were trying to get ranked anywhere, but almost never showed up in any results. It didn't take too long to see what was going on. It turns out that a developer had plugged in a regular expression to generate a canonical link on any given page. However, this regex was case-sensitive and was only whitelisting lowercased letters. So when you would go to the page "/job/12345/Data-Engineer", it would tell search engines to go to "/job/12345/ata-ngineer" instead. The link would still work since the site would use the "12345" portion as the page's ID, but nobody was searching for "ata ngineer". Literally just putting the letter "i" in the regex's flags was enough to start ranking on the first page for a large number of the company's listings (though reindexing took several weeks since the problem was large and existed for well over a year at that point).