I've recently embarked on a new map, and I know there's a pool here (thank you Toady). I'm digging my standard stairwell, when suddenly, I get a message that my miners were interrupted by a lizardman.
I know my stairwell is right next to the pool (damp stone), but I haven't even gotten the "you've discovered an underground pool" message. Therefore, I can only assume that the lizardman magically walked through the walls into my stairwell. The lizrdman was easily mobbed by the miners, but later on, I get a frogman, also randomly appearing in my fortress.
A giant Olm followed and killed one of my starting seven. By this time, I had dug into the pool from above, but then sealed it off. Somehow, they're still getting in.
Edit 1: Looking at the bug list again, I think the olm might have been pushed trough a grate by a flow (bug 680). It still doesn't explain the first lizardman, though.
Edit 2: I've placed cage traps in key funnels throught my fortress, in the hopes that one will catch a pool-dweller and tell me where they're coming in from. A month later a giant olm attacks. He didn't trigger any of them, which tells me that he didn't come from any of the water areas (fishing, farming, first breach of the pool) Still don't know where these things are coming from.
Edit 3: Found the problem. There was access, diagonally. A diagram to demonstrate:
Side View:
XX█
X█~
X-up/down stairs
█-solid rock
~underground pond
Still seems a little buggy to me, as the "you've discovered a cave pool" message didn't trigger with this, but that's how the game is played. I wonder if marksdwarves can shoot through such an opening.