Not having a job.
I'm extremely careful with my money, so the worst blunders I can really think of are buying a second copy of a book I already owned, or a game that I never got around to playing. I couldn't get a student loan, so that was never an issue, and nobody will give me credit cards, so I never had to worry about running up a stack of them. (I have one CC, but up until very recently, I paid it in full every month.)
I guess I spend too much money on books. I own hundreds I haven't yet read. But I plan on reading them all eventually.
Edit: I did let my ex live with me rent-free for 10 years so she could pay off all her debts and go through college, because I expected our relationship to last and that she'd get a good job and things would sort of balance out over the long run, but she left shortly after getting that job, so that never panned out. Cost myself about $50,000 in rent, lol.
For forty-seven years I've put up with it now. I must stop Christmas from coming ... but how?