About 20 years ago, a cousin of mine–10 years younger–coaxed me to rent a car for him (he was under 21 then, but only by a few months). He had a heavy date coming up and wanted a high-performance car; we went to the Avis agency in LAX and I rented a Trans-Am for him. He had the car with him during the day Saturday, and went to a baseball field a few miles away; I drove by the field, saw the car parked legally at the curb, and saw him on the field. (My older brother later castigated me for checking up on the cousin like that.)
The rest of the rental period was uneventful. The next morning he paid me for the rental and the gas, and I returned the car to the agency, coming home on the bus.
I later found out, to my shock, that he got a speeding ticket in that car! I hit the roof over this. But I perhaps deserved some blame for sneaking around the rule about age limits…
The cousin has since matured and doesn’t do things like that anymore. 
Post here something you did that you later mulled over and decided, in hindsight, it wasn’t such a good idea…
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Gee, it looks like we have one comedian too many among the Teeming Millions…
Against my better judgement I went walking out that door.
I smiled at one person, then I nodded to three more.
A man asked me for a dollar, I asked him what its for.
He said, “I have seen them!” I said, “Ok, it’s yours”.
Just about everything I’ve ever done has been against my better judgement.
–TIm
Stuff, and I think I will just have to leave it at that.