I've particularly noticed this happening for the Lair of the Fates (upgraded Chronos Lair) mob, but it has happened for other ones too (like the volano bat).
Effectively, a mob gets stuck off-screen on the left (not clickable) but can be snowball'd. Even when snowball'd, the mob doesn't disappear, so can be snowball'd repeatedly. This is nice occasionally, but it also prevents all other mobs from spawning until you switch areas.