A couple of well-known ones, and some others not so well known:
5 Fun (You need bashers this time): Run out of bashers
15 Fun (Don't let your eyes deceive you): Trailblaze a path to the rope trap, release the crowd, sit back and watch!
10 Tricky (There's a lot of them about): Well, that was easy.... oh no, wait, there's a whole other lot of lemmings the other side of the level!
(Actually, there's quite a lot of levels that one could apply to.)
15 Tricky (Ozone friendly lemmings): Forget that you need to save 60% and use all your bombers just for the hell of it.....
20 Tricky (One-way digging to freedom): Standard job with two athletes, front one blocks, second one turns round and bashes the wall. Only you've put your blocker on a thin bit of the platform.....
26 Taxing (Triple Trouble): Keep failing because you always get a lemming going the wrong way to build at the end
29 Mayhem (Save Me): Get all the way to the end, you've been very careful with your builders to make sure you have two left for the final turning bit. Oops, but you've only got one blocker! How did that happen?