A joke or tidbit meant for people knowledgeable in a certain field. The rest of the audience doesn't get it, but it's usually subtle enough for them not to care.
So just now I thought about the episode The Giant Bacteria and how an attempt was made to destroy one of the bacteria monsters with antibiotics. For a moment after the antibiotics were applied we see the outer "skin" (maybe cell wall, though the exact nature of the monsters is an unknown) of the bacteria monster become brittle:
As it turns out some antibiotics work by disrupting bacterial cell walls! And while they don't really work by making them brittle (it's more like causing them to burst if the bacteria tries to divide) I thought this could be a possible nod to actual biology.
I can think of more possible ones (watch Destructive Nature and you might find a subtle one), but I think I'll let the rest of you submit some so this thread doesn't become a one-man band.