People who show up late to choir practice and miss all the instructions given in the beginning.
That one guy who ALWAYS loses his music. I'm serious, there's a guy in my choir who can lose something that got handed to him five minutes prior.
Music that switches time signatures a lot. A piece starts in 4/4 and suddenly it's 2/4 for a measure, it goes back to 4/4 and suddenly it goes to 3/4 with a ton of syncopation and oh, don't forget the hemiola at the end! AAAAAAH! Once I learn the song it's FUN, but the rhythms are a pain during the learning process.
Hymns that are obviously written for one verse and the words and notes for every other verse have awkward ties, slurs or syncopated pickups that EVERYBODY misses because it goes against the rhythm your musical ear hears. It just sounds messy no matter what you do. It's even worse if there are tiny secondary notes mixed in and it's hard to tell which verse they belong to!
Songs with a really low soprano part, like the Gloria we're currently singing. It's ridiculously low! Really? We're sopranos, stop making us sing in the basement! Give that melody to the altos and let us sing a descant instead! I promise the song will still be pretty!
Part crossovers. Alleluia Ascendit Deus, I'm looking at YOU. You twist my brain! (But you're a fun song and I love singing you anyway.)
Blurry, xerox copied music.
When I choke on my spit in the middle of a song. Ick.
Instead of regular erasing where you rub off the pencil with the edge of a rubber eraser, this you just press down or mold into a shape to erase parts, without damaging the rest of the drawing. It's good for light erasing in the middle of a drawing, where you can't easily erase with a rubber eraser.