Biggest regret I've had was Le Skunk, my gf's 88 CRX Si.
Here it is at the one and only autocross it's ever attended in the 2.5 years since she's owned it. It's been disabled for at least 2 of those years with one problem or another, and has logged maybe 1000 miles - 20 in the past year...
It's just been one problem after another - distributor, suspension, engine replacement, clutch... She's got far more into the car than it's worth at this point, yet refuses to get rid of it so that maybe, just maybe, someday, she can actually get some use out of the investment. The longer it sits, the more things seem to go wrong.
My biggest regret on the matter is that it was partly upon my recommendation she got this car in the first place...
I have snagged some legitimately good deals. This was NOT one of them.