What slip was accidental brilliancy?

I've had an equivalent experience in over-the-board, face-to-face tournament play.
I played a move completely overlooking my opponent's tactical reply. When he pounced on it, I was stunned and horrified, and I had to sit there for more than 20 minutes with my clock ticking before I had calmed down enough to continue playing...
... and by then it had slowly dawned on me that my "blunder" was actually an excellent move, and that instead of losing I might already be winning.