When you need to piss really, really, bad and your boxers have shifted around so it's really hard to find the slit in your boxers that your cock comes out of. And the more and more you try to get your cock out the harder it gets.
The problem is compounded by several factors:
1)you are already semi-erect because you really, really have to piss (yes girls! sometimes this does happen)
2)If the pants you are wearing have a short zipper length
3)Or you are wearing older tighter jeans that you have been meaning to give to your younger brother/cousin/goodwill
...but are just to lazy/cheap
This is particulary bad when you are standing in a row of urnals with no dividers and you have an erect cock.
Someone might look out of the corner of their eye and think your gay
(its not something that you willingly talk about with your friends but) "Yeah dude, i was going to piss but...i had a hardcore boner. i guess i can wait."