Ran across this today, I find a separate joke in each panel:
The first panel pretty much sums up my views on the weekend's pending apocalypse (and velociraptors).
The second panel is a wonderful skewer of interfaith pluralism (which is my second-favorite type of pluralism).
The third panel I do not find the humor in. I think it may have something to do with mathematics or physics.
EDIT: Info on Error Bar. Now, fully informed, I can laugh at the third panel.
(cartoon via xkcd)