Constant interruptions, esp if it's the same person.
Keys that don't work quite right or having some oddity to the piano, like a squeaky pedal.
The phone, yes. I hate when people ask you to stop because of the phone, esp when they could take the call in another room.
Not quite a peeve, but a definite de-motivator. When you get a practice room right next to someone who is playing through their concerto. Dazzling playing, and then you get to sit their and
plunk out something you're just starting.
I hate it when people come up and talk to you while you're playing and expect an answer. Esp annoying was a boss who stopped me while playing to tell me what a good job I was doing. I think that was beyond a "peeve" then.
Out-of-tune pianos. Having the one extremely out-of-tune note be the tonic of the key your in.
Knowing when you're playing a piece that you could do better if the piano was better.
Students who walk by and plink the top or bottom keys on the piano. That annoys the heck of me, esp after I ask them not to.
Having someone standing behind you and not knowing exactly what's going on back there.
Having someone tell you it's good enough the way it is and you don't really have to practice it
anymore. The playing level may be the same, but there's a difference between playing comfortably and playing stressed.
How about having a practice room right next to the professor's office? I loved that one. No pressure there, and I think some of practicing should be experimentation.
Yes, not having energy to practice. Or not having recovered from the previous session. Attempting to practicing but having the play apparatus change so you can't do what you planned to do.
Having the building locked at college when it's supposed to be open. Having the little campus security people tell you there's a sports event going on so they can send anyone over to open the door that's supposed to be open. Having that happen on the day of a performance and having your music locked up inside in the building.
Having someone sing the next note when you're working out a piece. Of course you know the sound of the next pitch, but reading if off the paper while letting your brain think of the right fingering is different. They think they're helping.
Having people poke their face in front of the practice room door, just enough so you lose concentration but then they're gone.
Not having anything to cover up the window with.
Having someone fart or leave a BO smell in the practice room just before you. You walk in and discover the room is unusually warm... and then it also stinks. Ick. Then you can decide to stay in Mr. Stinky's room with the good piano, or you can go into a room with a crappy piano. Hmm.....
(Hint: You can use chapstick on your upper lip area to numb your nose to the smells.)
Practicing a piece so it's really down well and then discovering the performance isn't that great and you might have squeaked by with less work.
Having someone cancel a performance after you've worked on the accompaniment. I hate that when they don't pay or don't think they should pay because they couldn't do the performance. Grr....