What's the worst mistake people make in their 20s?

Additionally, What's the worst mistake you made in your 20s? or as a young adult in general?

I've watched a number of interviews recently and most of the guests say that they're happy the internet wasn't around when they were growing up because of all the things they did. I think the newer generations of today will probably have a bunch of regrets and dumb actions, partly because of the internet. Not saving money, thinking long term or procrastinating, college or not, travelling or not, etc. are some things that are usually listed.

What's the worst mistake people make in their 20s?
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