It kind of sounds like to the player the encounter was just... tedious and pointless and then your character just gets worse, and your only defense is to read the adventure before you play it?
Anyway, my confession is an adventure I made up myself and ran with a mix of first time players and veterans. The general layout is that some miners tunnel too deep, find a sort of ward containing a demon of greed and are then corrupted by the tainted gold that it produces, causing them to all become really paranoid and kill each other. The final encounter will be the demon itself, and if a fight breaks out, I have this interesting "save to not pick up the gold that spills out of the creature" mechanic planned.
The demon is then stuck at the bottom of the mine with noone to play with, and realizes that next time it should probably show some restraint. After all, it wants to either get free or attract more people to come play.
I sort of had this idea of a wild-west theme for the whole thing, so I went with what might be found in an old-west mine, and the players found some nitroglycerin on the way through. Most of them avoided it, but a couple grabbed some, and any time they did anything strenuous I had them roll to see if it went off.
Anyway, they meet the (disguised) demon and are naturally wary about it, since it seems trapped in a big ward, and all the miners seem to have gone nuts. None of them want to set foot within the ward. The demon meanwhile is trying to get them to accept some treasure.
One of the new players, having failed to convince the other players to shoot at it with ranged weapons, decides to start flinging the nitro at the demon.
Unfortunately they roll very poorly. I have them roll some dice to work out the scatter, and they manage to hit the party dwarf warrior pretty much in the face, hurting him and damaging most of the rest of the party too.
He is a bit put out, primarily because he thinks attacking is a bad idea, and secondly because he just got 'sploded. His words are something to the tune of "knock it off, or I might have to retaliate"
The thrower tries again. Comically he gets the exact same result. The dwarf retaliates with his axe, rolls a crit and instantly kills the thrower, taking him way, way into negatives.
The thrower takes this somewhat personally, and a real life argument breaks out. I call the session to an end and no one is interested in playing again because of it. The chill between the involved parties persists for several months.
Ouch.