What's the biggest mistake you made with investing, and only realized in hindsight?

Not the obvious ones, Not "I bought at the top" I mean the kind where you were wrong for months and genuinely thought you're fine.

I'll go first:

I bought Nike on the turnaround, brands still huge and were changing the product line and the strategy, and I figured I was in before everyone else believed it.

Before buying I wrote down what would make me wrong. Roughly, if the turnaround isn't showing up in the numbers after a few quarters, get out.

Then the quarters happened. Direct and digital kept sliding, digital is was down 10 quarters in a row, and direct down 6%. The turnaround kept getting pushed out further, and every time there was an excuse.

All of it sounded reasonable though

Then I reread what I'd written down before buying, and I'd already answered it, the exact situation I was explaining was on of my reasons to sell.

I hadn't changed my mind but I'd changed the rules for what counted as being wrong.

That stuck with me because I'd have done the same thing without noticing.

So now I lock the thesis before I buy, the claims, the rumors, and specific things that would prove me wrong. I set alerts on the levels that matter, and when earnings come out I check that transcript against my original claims which I have written down, what I get shown is what I write not what I remember writing.

I ended up losing on this position, but I'm happy that I realized sooner rather than later, I managed to sell at around 60, which is good because it now dropped to 40!

What's the mistake you only saw in hindsight, and what made you notice it?