What was your worst "I can do that. No need to pay someone else" story?

I’m getting ready to remodel my basement and I’m trying to be realistic about what I can actually handle versus what I should leave to the pros. I’ve done a few small DIY projects before, but this is a big step up. I want to learn from other people’s mistakes before I start tearing into things I shouldn’t.

What projects would you say are best to avoid unless you really know what you’re doing?