Doing it Backwards
There are few things that make you look as silly as holding a sign the wrong way. Everyone sees it, everyone sees you being oblivious to the fact that you are holding it wrong, and ultimately, you just look really silly.
In fact, we would say the oblivious nature of a person in such pictures is the most embarrassing part of all. So naive, so innocent: not even realizing that everyone is giggling at them, or just being clueless as to why everyone is laughing.