Any song containing the name "Willie" (i.e. most old English ones) usually get a stifled giggle from some one in the audience.
I like to sing "Ghost of Willie-o" (aka Bay of Biscay-o) but I have to close my eyes and ears because I know that the lines 'she saw her Willie standing" and "bring me back my Willie-o" will crack me up if I see anyone's expression while I'm singing.