On the PS1 version, which I think is the same as the Windows one programming-wise, you can also walk off the top of the screen. I believe you can also walk off the edge of the map, although I think the only level where it becomes an issue is Crazy 18.
There are a couple of places where builders don't work as expected: in particular, they will always turn around at a wall before you get the chance to bash through that wall, unless they also happen to be on their last step – ie when they transition to shrugger. So you have to be careful to place it at exactly the right distance. They also start removing steps if you get too close to the ceiling; I think on other versions the lemming just stops building. There are a couple of vids on youtube showing these.
As for the Mac version, I'm sure you'll all be familiar with everyone's ranting and raving about the Mayhem 26 graphical bugs. But there are a few glitches that I know it doesn't have:
• builders act exactly the same going left and right; you always get the chance to bash through a wall
• Nuke glitch doesn't exist; percentage is always calculated from the total number of lemmings. It surprised me that the DOS version has such a bug, since the goal is stored as a number rather than a percentage in the LVL format.
• I don't think pausing for time exists; on the DOS version, there are a pair of sounds that are always played, which are little beepy sounds corresponding to "Let's go!" and the hatch opening on other versions – and they're always played when the level starts even if you pause. On the Mac version, if you pause before the sounds are played, they won't play until you unpause. However, the Mac timer is incredibly lenient compared to other versions' timers.
• I don't think direct drop exists but I don't know what level to test it on.
• A lot of the steel glitches seem to exist, particularly the ones that stipulate that steel is destroyed by a skill such as a bomber if the lemming is not standing on steel. I don't know about the trigger-based ones, or the ones that rely on a blocker cancelling the steel area. However, my instinct seems to suggest to me that the Mac's steel detection algorithm is different from the DOS one, and I'm fairly sure it will detect steel earlier than the DOS version would.
• Diggers on the DOS version will carry on digging even if the digger itself seems to be floating over air, as the game checks all the pixels beneath the digger's tunnel. On the Mac version, there are either one or two pixels (ie DOS-sized pixels) which it doesn't check. This can be used for a quick glitch solution to We All Fall Down that uses 2 diggers (one releases the other, which leaves a tiny ledge), and can be replicated on the SNES and Genesis versions too, as far as I know.
• Bashers on the DOS version will always take an odd number of strokes, a caveat that isn't present on the Mac.
• Miners can mine through any one-way-wall (cf Taxing 13). I'm not sure about the other ones, but they definitely can break through floors and leave a one-pixel floor for other lemmings.
• Climbers climbing up to the top of the level can become trapped in the terrain – I can't think of an effective example at the moment, however. But I'm thinking about levels such as "Lemmings' Ark", which obviously you don't get on the Mac, but is an effective example of a level in which a climber must climb up to the ceiling in order to fall back. I don't think this would happen on the Mac; I think the lemming would become trapped. I'd have to confirm it, though.
• I think Miners and Bashers are much more likely to turn around at a Blocker than on other versions; on the DOS version, you can assign a miner right next to the blocker to release him, but I think on the Mac version the miner will invariably turn around before releasing the blocker. Therefore one has to be more careful when releasing blockers that the lemming doesn't turn around. For example, you might have to assign the miner quite far back, so that it is the top of his stroke that releases the blocker.