Monster Tactics against PCs - How have you misplayed them?

[reveal]

Morrus started a great thread here on scripted monster tactics. Some folks, myself included, posted how they've screwed up playing a monster against PCs because they've misread or forgot about monster abilities and the like. Rather than derail that thread, I'm starting this one so all the DMs, and players if you want, can post how they played a bad guy and, after it was over, realized they didn't use ability A, item B, or whatever that would have made them a much more effective opponent.

I'll start:

Bad guy has Haste cast on him. PC spellcaster casts Dispel Magic on bad guy. Bad guy has Haste dispelled. Bad guy dies. PCs loot body, find Ring and Identify it.

Me: "He has a Ring on Counterspelling with Dispel Magic in it."

Players: "Huh, wonder why Haste went away?"

Me:

 

[Savage Wombat]

I'm always forgetting defensive spells - mirror image, blade barrier, etc. Then my spellcasters get mobbed by the fighters.

I even had one EHP ready for this - he had Anti-Life Shell cast, but somehow didn't remember that it was up when he reached melee range.

 

[IronWolf]

I have forgotten a black dragon's fearful presence of all things. That encounter could have been much more fun had I remembered that.

 

[Kae'Yoss]

Our DM, when selecting monsters, didn't notice the frostworms abilty on death (20d6 damage or so). He only saw it when the thing died. After the breath weapon, we were weakened, and two characters dropped dead on the spot (one could not have survived that even if he had made his save)

 

[Andre]

Our DM, when selecting monsters, didn't notice the frostworms abilty on death (20d6 damage or so). He only saw it when the thing died. After the breath weapon, we were weakened, and two characters dropped dead on the spot (one could not have survived that even if he had made his save)

Interesting. My group had a similar encounter. The GM threw a dozen worgs at us. After we'd killed about half, he threw a frost worm at us. By the time we killed it, everyone was injured and most characters blew their saves. Result: TPK.

I think if the GM had realized how deadly the worm would be upon its death, he would have used something else.

 

[DarrenGMiller]

I forget things like Frightful Presence and Stench.

DM

 

[ThirdWizard]

I made some huge mistakes recently with an undead creature. First of all, I missed its Fast Healing 10 through the entire battle. Halfway through the encounter, I also realized it had DR 5/silver or good. I admitted my mistake to the players and just told them about it. *sigh*

On the flipside once, long long ago, I forgot to give saving throws to PCs hit by a ghoul's paralysis attack. The elf killed eventually the ghouls while everyone else kinda stood around. Oopsie. At least I didn't kill them all off with that one.

 

[Graf]

NPC spell use is the toughest. With NPCs I've created its not an issue, but otherwise it can take a lot of time. Especially if its something like Druids where they have lots of specific use spells (heat metal, warp wood, produce flame).