Blocking creatures with Menace with a single creature doesn't give any warning, just undoes your block and continues to combat damage.

There should be some form of 'illegal block' warning or something that prevents you from attempting to block creatures with Menace but not actually blocking them; my friend was being attacked by a 9/9 with Menace, but had plenty of indestructible blockers to block it. Because he didn't realize the attackers had menace, he only blocked with one creature. When he clicked done, the block was undone (for being illegal) but he didn't get an opportunity to correct it.