Words or sayings for that moment when your underpants fall apart while you are wearing them

There are many strange, awkward, or underacknowledged moments of life that are in need of a word or phrase by which we are able to express their commonality. One such is that awkward moment when your underpants fall apart while you are in the middle of wearing them. You may smile and put up a brave face, chat pleasantly to your boss as if nothing out of the ordinary has happened, but in that moment, something fundamental has shifted in reality; your underpants have changed into something completely different. Let's workshop a few phrases for this important moment now:

your unders down under go under

asunderpants/gone asunderpants/asunderpanted: "He smiled the smile of one who had gone asunderpants". 

wedgied by the nothing

ununderpanted

falling into the abyss/fell into the abyss

to take the invisible plunge

going silently commando

wearing one's blunderpants

biology meets topology

knickering off

your clothes clothes take their clothes off

underdance/doing the underdance

to have your knickers knock off and your knackers nick off through a nook in your knickers

to have a derriere in the merrier area/to have a derriere make merry in the nefarious area

going hey down derriere

when it's over for your unders.

Please feel free to leave further suggestions in comments!