Yesterday I had a won game in a hard-fought game. When I played my Queen on an undefended square where it could be captured by the opponent's rook, he resigned! (he saw the mate next move but missed the rook capture...).

Sounds lucky, but why not put the game up so we can see it?

Here is the full game. Analysis with the help of Stockfish on the Deep Fritz 12 GUI.

WOW ...enough said

Yeah, I did the same thing once.  At first glance, only an idiot would do such a thing.  But you have to consider the time element.  In my case, what happened was I offered a queen trade, and two days later the guy took my queen with his queen.  But two days later, my mind had moved on, and I completely forgot the context, and I was startled to see this guy's queen on my king, not realizing that I could just take it back.  I moved my king away to safety, then a minute later remembered, and then I resigned.