Annoying washroom habits of people:
-People who awkwardly explain their need to use the washroom.
-People who use the phrase "stink it up" in a civil setting, either referring to their unpleasant odors left behind or used in a plea to leave it in a suitable manner for them to make use of following yourself.
-People who don't flush. It's not just the unpleasantness of it all, but if you're at a someone's house, sometimes it takes so long before you can flush it again after flushing for the previous person that you're stuck with the choice of waiting it out when you're in a hurry or repeating the same offense.