Just putting this info here in case anyone needs the details. On the first game, there are 101 (not 100 as usually claimed) levels that can be solved 100%, glitch-free. The extra one is Mary Poppins' Land, which can be done by stacking bridges to make a wall to hold the crowd. Two more can be done with glitches: Compression Method 1 (steel removal glitch, doesn't work on the Mac version) and The Boiler Room (sliding glitch).

One of the Sega Genesis levels, "A trap's a trap", also requires the use of the nuke to complete it conventionally, as you don't have enough bombers to break through the roof above the exit (the lemmings being massed together in the pit helps to ensure about half of them make it to the exit before exploding)

Not true. You can time your bombers so they explode while falling off the starting platform, effectively eroding away its top corner. This allows you to either land safely on the platform below, then use one blocker to send the rest to the bottom exit; or else build across to the top exit.

Removed natter (from Video Game Caring Potential):

• In versions where Blockers could not be reassigned, it was impossible to get 100% on levels that required Blockers; you HAD to sacrifice them to end the level (except where you could undermine them, at which point they'd revert to normal).

I took out the Unwinnable comment for 2-player SNES Lemmings, because it's not actually true. Originally, the article mentioned that the Nuke option is taken out in 2-player mode making the game Unwinnable if you have blockers or otherwise trapped lemmings somewhere on the map. The Nuke option is still available, but hidden, and I don't think the manual even mentions this. Player 1 and Player 2 have to both move their cursors to the bottom center of the screen and the Nuke icon will appear in the middle. Both then have to double-click to blow everybody up, just like launching a nuke on a submarine! They obviously did this so that one player couldn't just kill everybody's lemmings in frustration...