Several times, with the DOS version of 1.2.0 prerelease 1, I've observed a monster that doesn't see invisible and doesn't know about my presence attack me when I was invisible instead of remaining where it is and generating the message, "The foo seems to search for someone or something." The turn after this happens, the monster returns to its normal behavior. I have yet to see this happen in a non-corridor location. I possess screenshots that demonstrate this bug and can share them if requested.