I know for myself I usually get caught up learning a completely new language. For instance, in the last GGJ I ended up making a game in C# on XNA and ended up barely being able to provide help for my team. However, when on game jams with CSS3 and Javascript at my disposal I can create some awesome things.
"Hack first, learn second. Dont try to learn a new language, instead, hack something together with what you already know."
It probably depends on what you want to get out of the hackathon, but I find most of the fun being gaining familiarity with cool new APIs.