RIP, about 1/2 gallon of Kolsch. Also, RIP the rest of my morning while I clean this mess out of my kegerator. The air circulation fan was splashing it *everywhere*, which means a complete teardown.
It was a bad o-ring on the beer post of the keg. I'm usually pretty good about checking them and hitting them with a little keg lube, must have gotten sloppy about it this time. Doh.
All three kegs got both o-rings replaced; paranoia has now set in regarding leaks.