The most disturbing thing I heard once was an apparently genuine emergency communication on this frequency between a ship on the North Sea and an approaching helicopter that was stepped on by some numbnuts every few seconds with "you´re on GUAAA--AAAA-RD!".
Why it is not possible to just ignore the inadvertent calls on 121,5 and let the initiator notice on his own that the lack of a response is due to his frequency selector is rather beyond me. Ignoring things that are not meant for one is a long-lost art, it seems...