My office's petty battle-starters:
Kitchen behavior. I could fuel the website Passive Aggressive Notes solely on the Post-Its that pop on in my office's kitchen. Half the office seems to become infuriated when the kitchen is not sparkling clean, the other half thinks magical fairy-maids come by to do their dishes, throw away their empty wrappers, and refill the coffeepot/tea kettle.
The candy jar. Woe betide you if you're having a vicious chocolate craving, for the Candy Watchers will loudly berate you if you hit the communal jar hard one day. (Though I've never run afoul of the Watchers, I did get a kick out of one Watcher berating one candy-loving coworker, saying "Those things will give you a heart attack!" before popping out on her smoke break.)
Arrival/departure times. This battle happens every time the office is collectively stressed out by the higher-ups' demand for overtime and working through lunch breaks. Some people will watch their fellow coworkers' arrival/departure/lunch break times like a hawk and loudly comment on them. I think it's a way to defend themselves if the higher-ups call them out on not working hard enough, so they can retort "So and so doesn't work through his lunch, AND he arrives 2 minutes late everyday!" It turns normally reasonable people into insufferable neurotics. I was very surprised one time when it went as far as one coworker altering another's timesheet behind her back to reflect a time that he felt was more accurate to her comings and goings. (He did get chastized by the boss for that, but not formally disciplined.) When the stress of all this seems just about to become unbearable, the coworkers enter into "I don't give a flip anymore" mode, bring in donuts for breakfast or go out for margaritas at lunch, then become calm and agreeable again. Until the next time.