I don't think I've ever seen this happen, on either the Trebek or Narz versions.
I'm watching one of the Saturday reruns on Buzzr. The puzzle is "Too Good to be True", and it came down to the last set of matching prizes. With the board completely clear, the first contestant guessed incorrectly. Her opponent then got a chance to guess, and was also wrong.
Alex begins to explain the symbols on the top part of the puzzle, prompting the second contestant to buzz in and finally guess the puzzle. Has this ever happened before? I would think they would've thrown the puzzle out, start a new match, or just edit out the match altogether.