Buying Estate with $3 or $4 in the very late game when Tunnel is on the board. Buying Duchy with $6 when Fairgrounds is on the board (and would be worth 4 VP for me).
This happen to me less and less. But what happens to me: Buying Estate when Great Hall is available.
I've done this before on purpose.
Yeah, of course, me too. Here I was ignoring Golem, Baron, three-piling and other edge cases.