This happens sometimes when you're playing a person, more often when playing the computer it seems. The lichess Android app for e.g., on the easiest setting, always follows Nc3 this way (usually following on the first variation below).
[FEN ""] 1. e4 e5 2. Nc3 Bb4 3. a3 ( 3... Bxc3 4. bxc3 Nf6 5. Rb1) (3... Ba5 4. b4 Bb6)
Every time I see this played I wonder what black is trying to gain here. He either ends exchanging the bishop for my knight (and giving my rook a good open file) or locking his bishop in one corner, neither seems particularly useful.
Am I missing something?