Years ago I spent a lot of time on a consumer finance site. It was an off-shoot of another niche finance site I'd made that was performing amazingly well on Google for a super competitive keyword.
So, I'm working on this more ambitious consumer finance site; I've got the branding down, I have excellent content, I have a lot of regional data that was being dynamically produced based on a users' search - it was really smart and that same technique is what was helping my niche site do so well.
But the issue was that this consumer finance site would not rank. I kept refining things, adding more content, improving the UX, and of course, building great backlinks. I was getting really creative with the ways that I acquired them.
After months and months of work and not seeing it rank at all, I started to think I must have gotten a penalty or something. I started to move away from this project and instead focused on something else.
Maybe a year later I revisited the site and I just happened to look at the source code when I saw rel="noindex" in the <head> tag.
I could not f***ing believe it!!
I had checked my source code so often, searching "index" and seeing rel="index" in there...
What I had done was added two separate lines far enough away from each other in the head tag that you wouldn't notice by glancing at it.
One tag said "index" and the other said "noindex", and Google was honoring "noindex".
That is definitely my biggest mistake. I felt like such an absolute idiot after discovering that.
I suppose it was bittersweet, too, because the fact that my site wouldn't rank had shaken me a bit at the time. I thought myself so knowledgeable of SEO and here I was struggling like a noob. My confidence was knocked.
After realising my dumb mistake, I got back on top of my game.