Teachers who can't teach.
Especially when they clearly know what they are teaching as well. I have a few teachers at college like this, great for one to one help on something, but no hope when trying to communicate a concept for 10+ people....
The head of the course REALLY grinds my gears, since he is fantastic at running it the course and should stick just to that. But he doesn't, he takes a lot of the classes too. Of those, he chooses to take the most important subjects too (maths and business management, for example). He has written books on maths and engineering, and used to be a university level lecturer (how I don't know).
Thats his major problem though, in that something he finds simple is a major problem for someone learning the subject. He skips past it (often having calculated answers in his head) without explaining it, leaving the class guessing what on earth he is doing. He writes at an INCREDIBLE pace which is hard to keep up with, let alone think about what is being said, and on top of this he then makes errors throughout since he is going too quickly, relying on his notes and quick solving of problems in his head without checking. This makes for a VERY confused class, and often I have left lessons knowing less than when I walked in!
One of the worst examples for grinding my gears was a year ago when I submitted a maths assignment to him early. A week before deadline he throws it back at me, tells me I've failed as it is but not why. When I ask, he just tells me to read Steven's (a guy on my course) and I'll see it. I read through Steven's (thinking to myself that this is surely plagiarism...) and find I've done far more work, and far more detailed work at that. So I ask him for another hint as to why I've failed. "you have shown a lack of knowledge dealing with the electrical system, your solution is wrong".
I still couldn't see what was wrong, but asked Steven directly now. Put simply, I had worked out exact component values to make the circuit work as expected. This had taken PAGES of work to do, but was clearly stated on the assignment. I finally passed the assignment with an exceptionally high grade... by removing all of these pages, and adding a sentance that said "x component needs to be higher or y component needs to be lower". I was furious, hours of work to figure EXACT component values (as you would do in real world circuit designs) had failed my assignment on the grounds of not understanding it...