Silly names and blatant metagaming are on my list, as well as one that cropped up just recently: supercompetance and/or ridiculous levels of development by NPCs.  Allow me to explain - 
FR campaign, we're pretty much the head honchos of this small (and I do mean small... talkin' 2 towns total) island kingdomlet in the Sea of Fallen Stars.  On this island there's a mountain full o' diamond deposits, so we're small, but we're wealthy.  Export deals with Sembia and Cormyr, and a small Thayvian enclave who're interested in our product for spell components.  All well and good.  We're away from home for about 3, 3 and a half weeks in Sembia doing the whole trade renegotiation thing, and upon returning we find out that we have a Problem.  Capital-letter 'p' grade of problem to the tune of .. well, he's something.  A vampire who may or may not be psionic, and has a crew of brainwashed scallywags.
Enter, the Vampirate.  We're trying to deal with this guy, who has since gone to ground in our port town.  Which is to say, the only town we have that isn't the nations capitol.  We do the natural thing and set about trying to Deal With The Situation using the only method we know how; blundering interaction, blind luck, and violence.
Now herein lies my problem - Dread Captain Mindfang here has had at most less than a month in which to entrench himself in our quiet beach community.  In that time he and his minions have carved themselves an extensive underground lair, tunneling thru the bedrock and connecting a large number of warehouse sub-levels, then filling it all with traps and stashing their booty there.
And we know there wasn't anything like that before we left, because we friggin' built that town in the first place.  When we left for Sembia there was no Batcave under the shipping  district.  Not to mention the fact that they did all this significant construction without anyone in the town catching a single whif that something might be up.  
In three weeks.
There's more, but you get the gist.  That sorta thing breaks my suspension of disbelief like dry twigs over a lumberjack's knee.  It's one thing to have set up shop incognito somewhere, it's another entirely to have dug the great dungeon of Underwarehouse in our own home without us catching wise.