Maybe not the funniest drug experience, but something that came to mind that happened about 10-15 years ago.
Two friends and I were having fun with some RC-drugs, we were taking analogues of coke and speed and smoking early forms of spice (JWH-018).
One of my friends had duties to leave piss samples every week, so we had a few detection kits, so he stood up and took a mug and pissed in it, sat back down in the couch then used a kit, no problems, nothing is detected, great!
Few hours pass.. We were snorting some analogues, and my friend who had just pissed earlier into a mug.. felt the disgusting analogue taste dripping into his throat. So he grabbed a mug on the table by the couch.. and downed it.
He flew up from the couch and he became a piss fountain, spraying piss everywhere from his mouth, he retched and he cried out "ITS MY PISS, I DRANK MY OWN PISS!", he kept retching, running around the couch and the table while half crying. Meanwhile my friend and I were laughing ourself to death until we couldn't breathe, good times!