My biggest pet peeve is simply this: inconsistent (and often incorrect) information from CSRs, whether they be phone agents, ticketing agents, gate agents, FAs, etc. This is especially a problem with United, but I'm sure it affects every airline to some extent (for example, see this thread regarding TATL in-cabin pets on USAir). Most of the time it's a minor irritation, but (as with the linked thread) it can sometimes cause major problems, and it's really irritating if the airline doesn't hold itself accountable for one of its agents providing incorrect information.

There is no good workaround for this except to ask the same question of 3-10 people and then hope that the most common answer is the correct one... unfortunately, that is both inconvenient and unreliable.

The TRUE solution would be for airlines to ensure that their CSRs are well-versed in all aspects of policy, and to actually look things up when questions arise. I would much rather hear a CSR say, "I don't know, sir, would you mind holding while I do some research?" than get a confident but entirely incorrect answer. We FTers are generally savvy enough to recognize wrong information when we hear it, but most flyers just take an agent's word for it... and I shudder to think at the problems they endure when that word turns out to be wrong.