Monsters that GMs Don't Like to use at the Table

I was reading up on a game forum where a GM had what he thought was a dastardly plan for surprising the party with a monster. The plan was a decent one, and kind of amusing, but the discussion took a bit of a dark tone to me and went off in a bit of a "screw the players over" sort of fashion.

My mind immediately jumped to a "monster" that I pretty much refuse to use and it made me wonder if other GMs have monsters that are pretty much on a banned list for their campaigns/tables.

For me the one monster I rarely use is Green Slime. I hate Green Slime because it is barely one step removed from simply having a lightning bolt fry a PC from out of the blue. Think about it....Green Slime is pretty much a stupid ambush predator that (in many systems) drops down from above and instantly starts destroying any organic material it comes in contact with. If a PC is lucky maybe it'll just take out an arm before it gets lopped off.

Oh hell no.....not in my game.

Don't get me wrong, I have used Green Slime before, but I always give the PCs plenty of warning before hand. They get to see the Green Slime long before it has a chance to attack. To me, Green Slime is more of an obstacle encounter than a monster encounter. I've used this creature twice and it was only an issue for the PCs the 1st time and that was all on the players.....

In my HackMaster 4th Edition game I had an "evil" Dark Elf Necromancer. I ended up having to give the PC the misguided quirk because the player, a genuinely nice guy, had issues playing a Neutral Evil PC.....his own real-world personality kept creeping through. At one point in an adventure the group had found a spellbook stowed underneath the cook stove in the ruin's kitchen. There appeared to be a mass of Green Slime attempting to devour the spellbook, but it was being kept at bay by some mysterious force. Basically this was loot I wanted the Necromancer to get, but I wasn't just tossing it at him.

The PC grabs a stick and tries to get the Green Slime to attack the stick instead of going for the book. Now I had already figured that if the PC simply used the stick to push the spellbook away from the Green Slime that'd work, but the player couldn't figure out that action. At one point I put a phone book under my living room couch and handed the player a broomstick and asked him to show me what he was doing......

....eventually the player just got frustrated poking at the phonebook and just reached in and grabbed it.

In the next few second in-game the Necromancer was attacked by Green Slime and the Ranger managed to amputate the Necromancer's arm at the elbow and staunch the bleeding before there was a loss of life.

Even though this was a good session for me.....still not up to using Green Slime.

Are there any particular monsters you just refuse to use, and why?