In terms of spending money on art, I always say that you never regret the art you buy, you regret the art you DON'T buy. There are several pieces that I totally regret not buying at the time, although, in fairness- I probably didn't buy them because I didn't have any money. The worst one in recent memory was a teeny painting I saw in an antique mall for $500...it was of a spooked horse running from a lightning bolt. As I looked at it I noticed it had a decrepit tag on it saying "Fuseli". I asked the dealer and they said it was "attributed" to Fuseli but there was no authentication. It also looked to be cut from a larger painting and then framed. So- most likely it was not a Fuseli, and I didn't buy it, even though I REALLY liked it. After I left I changed my mind and went to get it and it was gone. So I blew it on that one.
Henri Fuseli- The Nightmare