This has happened quite a few times to me. I was just scrolling around, watching my underground kingdom, and suddenly the mentor tells me that the gods accepted my sacrifice. What sacrifice? I had no idea.
When this first happened I thought it may be a bug or something and forgot about it quickly. But every once in a while something kept falling in my temple pool.
I scrolled to the temple and saw a wizard kneeling right next to the hand in the center of my temple. For some reason he must have jumped (or been pushed) in the pool. I figured that after a while he would just be sacrificed.
Is there any way to prevent this? I can't let my precious vampires (yeah who cares about wizards) near a place which is that dangerous. Currently I lock my bigger temples and for praying purposes I build temples without a pool.