According to Andy Soltis, Fischer had prepared a very similar line, in which the queen was on a different square (maybe h3 or f3). At the board, he had a mental glitch, forgetting for a split second that 12...h6 could be met with 13. Bxh6; he only remembered just after he'd picked up the h-pawn, so the irrelevant 12...h5? was, under the circumstances, the best move he could play! However, it basically wastes a tempo, on top of weakening the K-side badly, so against a grandmaster as strong as Unzicker, it was just too much.