Student or Learner
Jerry has the best nervous twitch of all, which is when things get really serious he gets really comic. So he's always pulling the carpet out from under you if you start to take yourself too seriously.
Does it mean: When the situation is really tensed Jerry is the one who acts most emotionally of all of us. He starts to behave in a funny way and tries to thwart our plans if we became too serious.
Cambridge Dictionaries Online - Cambridge University Press
Fig. to make someone or someone's plans fall through; to upset someone's plans.