I have brewed some 20l batches where I have kegged half in a 9l corny keg, and bottled the rest with sugar for priming (plain white sucrose). In a couple of these batches the kegged beer has been fantastic, but the bottles have a chemical/fusel-like flavor, and in one batch quinine-like bitterness. None of these flavors were apparent in the kegs, even after several weeks.
The bottles have generally been stored in room temperature (20-21°C) for about two weeks after priming, and then stored cold (3-6°C).
What could be the cause of these off-flavors?
Edit: Thanks for the answers. I was pretty sure that the beer wasn't infected, but I guess I have to accept that I probably was a bit sloppy at some point :-)