A guy in our group drove our enjoyment of Speaker in Dreams into the ground hard enough to cause a spiderweb of cracks, and then stomped on it repeatedly without scraping the horse poopie off his boots first.
Fortunately, it was a single module, and we just haven't invited him to DM again since.
The problem in talking to him is that he lacks some fundamental ability to interact on an emotional level with other people. He doesn't get polite comments. He makes statements in a bold, declarative voice that don't really relate to the situation at all. He's not rude -- he's actually a genuinely nice guy -- but there's some kind of social-cue circuit that most of us have, and he ain't got it.
In fact, that was half of what was wrong with his DMing style. We'd hit an attacker for 12 points damage and say, "Does he look badly injured or just scratched?" and he would say, "The ___ does not look happy to have been hit by your ax," with this big smile, as though that was explaining something, or as though he were being clever. I know that some monsters need secrecy regarding their hit points -- you shouldn't always know how damaged a specter is -- but as the usual DM, I knew what the monsters were already, and I knew that they were grunts with maybe 2HD apiece unless they had class levels. So we should have been told whether 12 points of damage was a tiny graze or a stab to the gut.
The other bit that was wrong was when he either didn't read or didn't understand the module. I've complained about this before, but basically, there was one big powerful monster that we might have met in three places, and he had us meet the monster in all three places as separate monsters. And in one place, he let us unload all our big guns (highest-level spells, limited abilities, most powerful magic ammo, etc) and then arbitrarily declared that it didn't work -- because he wanted to give us a cliffhanger. (It was a cliffhanger, later, with us trying to survive with most of our good stuff used for the day because he didn't feel like having us fight that session.)
And the other other bit was his railroading. I was shocked to hear from other people that Speaker is one of the least railroady modules out there. When we realized that none of us had the right material to hurt a certain bad guy, we went looking for a magic shop -- and were informed that it was closed. We knocked. No answer. We pounded. No answer. We prepared to break in (we were saving the town, after all), and were informed that the magic shop owner had gone home and taken the entirety of his stock with him. Later, when we tried to get into a specific house, we were told that we couldn't break down the portcullis... despite the dwarf with the magical weaponry, Power Attack, and the wand of Silence to hide the racket. In fact, the DM held up his hand and said, "This is the hand of plot, telling you that you can't go in there until you go to this other place first." Then we went to the other place, and did the end battle FIRST because he, the DM, forgot to draw the side-rooms we'd specifically asked about on the map we had to use.
So railroading to an unpleasant degree, and an inability to convey or receive information in a reasonable manner. Not a good mix. Glad it was just a one-shot.