Many 'gotchas' worth looking out for, including:
Not having your tools ready
Trying to learn a new language or API
Going without sleep
Not going outside or taking a break
Not eating properly
Many first time Jammers make the mistake of diving into programming before getting their tools in order. When a Jam begins, you should already be familiar with your compiler, editor, and language.
You should have already installed the tools that you need to program the game and create the art. The game engine that you want to use should have already been downloaded, and you should have already tried making a simple tech demo or "hello world" example.
This preparatory stage is essential, since 48 hours is not very much time. Stories of entire days ...