I guess selling a guitar that you loved, because you remember that actual guitar, the way it played. And every time you remember it you wince and think "dammit, why did I do that?" :x :x
But there have been a few times I've wanted to buy limited edition guitars, then they suddenly vanish from the shops. There was an SG Standard Korina very briefly available in the early 90s, and I wish I'd bought it. Still kick myself every time I think about it.
And another thing - selling a guitar for less than it's worth! I had a 1964 SG which I stupidly got talked into selling for a pathetic price. That was the end of dabbling in the vintage market for me.