A bit of a hybrid between a double entendre and an unintended pun, and in the best examples, a faux pas as well. A phrase, word or action which in a given, hopefullly public, context has an unintended (usually hilarious and/or offensive and inappropriate) meaning.
1. He was telling the checkout chick that the day was looking up when all he had in his basket was a packet of kids party hats and a box of condoms; what an unentendre!
2. Asking someone 'is that guy retarded or what?' to find out that they *are* retarded and that they are said someone's brother.
3. Saying 'what a lovely pair' to another father whose teenage daughter was playing with yours - only now his daughter is alone, making you look like a complete pedo.