I don't employ anyone, but I do deal with the end product a lot, so the following may be a good starter:
1. As you've identified: bleeds.
On a good day I can expect to spend about an hour fixing/adding bleeds!
2. Not paying attention to requirements.
When I agree a job with someone I'll send them a full artwork spec with everything I need in terms of resolution, bleed, marks, format etc. It's super frustrating to ask for a high res jpg, with 3mm bleed, at 300dpi, in cmyk colour, and get a png back. Which leads me neatly on to....
3. Giving up on the job when you hit send.
So many clients or their agents will send something on to me that's not right and then either ignore or reply late to my emails telling them what's wrong and how to fix it. There also seem to be a lot of GDs out there who simply refuse to admit that they've not done something right. While I can appreciate professional pride this only hurts the end user!
4. Being rude.
Again, more common than you'd imagine. It's a high pressure industry, but there's no need to be a bastard about it (not that I think you would be, but you know...)
5. Not being able to match my tea to the pantone I've requested.
Sorry - I couldn't resist!