Has this glitch been discovered yet?
Lemmings 2 Glitch, see attachment.
The glitch may have something to do with the 16x8 tiles thing, but I don't know. Either way, it's interesting.
I just tried to reproduce this, but without success. I managed to get the lemmings a position identical to those in your pic, but the trapdoor appeared normal. Strangely, while the lemmings were all in the same positions in my game, their animation frames were different - the blocker was looking the other way, the two lemmings on the right had their arms in different positions (meaning their animations were 4 frames off compared to yours), and the crowd on the left looked different too (perhaps drawn in the opposite order?) I'm using the "fixed" version of L2, maybe that has something to do with it.
And while I was testing this, I found something even weirder. I set the blocker to explode, and while it was counting down, the two lemmings on the right struck an invisible wall (which didn't exist before or after this occurence) and turned around. See attached pic: note that the blocker is counting down to explode, and the lemmings on the right are spaced exactly the same as in Dullstar's pic.
I have managed to reproduce this behaviour on another level (Classic 1). The key to making it work seems to be the basher. If you set the blocker, use a basher (anywhere seems to work) and then explode the blocker, then there's a chance you might notice something odd.