C: Christianity must be true. It has nearly 2 billion followers
A: So what? That's just argumentum ad populum. Just because lots of people believe it, doesn't mean it's true
C: Does so! [leaves in a huff]
----- some time later -----
C: Catholics aren't real christians
A: I thought you said earlier than christianity had two billion followers?
C: I did. So?
A: If catholics aren't christians, then it doesn't. Dummy.
C: *wha?*
(no, really. I've seen these two arguments from the same "apologists" within short periods of time. Cognitive dissonance writ large)