Dog Moon said:
For some reason, they're almost always beating down the PCs when I go 'Oh yeah, TOTALLY forgot about that!' The first time I did that, my players just like stared at me or something.
I often do that. On purpose. When, after one or two rounds, they still haven't harmed the enemy in any significant way, I say something like: "Oh, I forgot it can do that." and laugh my evil DM laugh.
But let's see: I myself described to the group the BBEG from the final encounter (which they didn't think would be right there, right then), and somehow forgot to mention that it was just a painting on the wall. It was like someone pushed the PANIC button.
Then, another DM had a couple very funny fux-pas(es?) we keep teasing him about even 5 years later: First, we heard that our home town would be attacked soon. When we asked how long it would take us to get there, he said "2 1/2 days if you hurry, otherwise 3". Of course, we went with 3, so we wouldn't be too exhausted. That threw off his plans. While we were supposed to be there just in time to witness the start of the attack in the evening, we arrived at noon and the siege was well underway. We fought our way through some enemies and finally ancountered the leader: A vampire cleric of Velsharoon. After we fought and destroyed it, and played on for about 5 minutes, it dawned on us: "Did we just fight a vampire in broad daylight???". The encounter was supposed to take place the night before, and noone remembered that little detail until after the fight - DM included.
In the same vein, we were (much later) in some city, and engaging in some late-night drinking in a tavern. Our druids, when asked what he'd do with his dire bear (which wouldn't fit through the door, and anyway, they might have something against dire bears in taverns...), he just said he'd leave it in front of the tavern.
Later that night, the player asked about the bear, what it was doing. DM: "Oh, I guess it ate some kids that were playing on the street." Of course, I had to ask: "Why are there kids playing on the streets at 2 frikkin' AM?" At least that cancelled out the daywalker.
The worst we had so far was the DM (not the daywalker-DM) throwing a frost worm at us (I think it was a frost worm, something like that in any case). He didn't see the death burst ability until the thing died. Noone made a check high enough to know about the death burst, so we were all right beside the thing. Many of us have also been subject to its breath weapon before, so we were not exactly fresh. It killed the party mage (he never even had a chance, the damage was far too high) and my character (because I failed the save). Of course, after he realized that he gave us an encounter that some of us could never have survived, he let us be resurrected without cost, but my character - being a shapechanger, which noone else knew until it reverted to its true form upon death - lost the main fun element at this point.