Haha, great thread!
Fortunately I'm lucky that 150% of my flight time has been in airplanes with a lavatory. So I don't personally have a grumper story of my own doing. The closest I've come is when I opened a 20 oz. Coca-Cola at 9000 feet with one hand while another was on the yoke... couldn't react fast enough. Had a massive brown stain on my khakis for the next two FBO stops. Tried explaining to the cute girls behind the desk, but I'm sure they had a laugh as I left. Oh well.
This one happened to a captain buddy of mine at my former company. Coming in from a 1.5 hr leg, about 10 minutes from landing, FO flying. He's sweating his balls off trying to keep in a massive steamer. Finally, he just has to give it up and hand over the controls to keep his O-ring in check. So captain flies it in, taxis in quicky to the gate, and the FO is out the door practically before the props are done spinning. Bolts for the crew lounge lav (why not use the airplane? I have no idea). So, the captain shuts it all down, deplanes the passengers, and wanders inside to piss. Well he goes into the loo and the place absolutely reeks. So being the smart ass he is, he says "Dude, you smell like a f**king sewer!" From behind the closed stall door, one of our black rampers exclaims "Who you talkin' to muthaf**cker!!" And like that *poof*, the captain was gone.
Another time, two hour flight heading down south. Shootin' the breeze the captain about women, beer, sports, or somethin', when he gets this look of absolute fright on his face. I'm like "what's wrong?" He just hits the cabin call button. I'm a little confused at this point. Then he says, "Hey dude, I'll be right back." So he swaps out, and me and the F/A are just hanging out wondering what the heck is up. So he calls forward, and he swaps back up. I'm like, "What was that about?" He says, "Holy sh*t dude, I just farted and thought I shat myself!" I couldn't breath I was laughing so hard. The sense of relief on his face... priceless. I had to tell the F/A later when the flight was done... she goes, "yeah, I figured!"