My number 3 would be when the gym is too crowded. Since the gym I attend is very small, some times there is a line to do certain workouts. And if you're on a time schedule then you might not get to finish your workout.
MY number 2 goes along with number 3, Limited equipment. Small gym = limited equipment. Limited equipment = longer lines on a crowded day.
And my number 1 would have to be being hit on or picked up at the gym. I am not the type of girl that's there to stand by the water fountain and flirt. Believe it or not, I'm actually there to workout. Am I saying don't approach me? No, not at all. You wanna say hi or ask for a spotter or give me constructive criticism on proper form? That's fine. But if you want to have a conversation at least wait until I've finished my workout, don't interrupt me while i'm bench pressing to just chit chat. I didn't come to mingle. I came to lift. As i'm sure you did as well, so go do you and let me finish my reps!