I heard this story a couple years ago. Cant remember who told it to me. If someone recognizes his story, he can claim it.
A freighter crew (maybe DC-8) always had a cooler full of ice installed for them in the galley behind the cockpit, and a case of soft drinks next to the cooler. Often there would be too much ice in the cooler, leaving no room to put the drinks, so the routine was to rip one or two of the ice bags open and poor the ice down the chemical toilet.
One day though, the new crew member who was tasked with making room in the cooler did not notice that on that particular day, the bags did not contain ice but dry ice.
It seems that dry ice and chemical toilets don't get along very well and that some sort of foaming/sputtering/smoke quickly filled the toilet compartment and came gushing out the door as the frightened crew member retreated to the cockpit.
It came close to an emergency descent.......