Quite a while back (I've been busy), we had a mock-exam practical test in the chemistry lab, and we had ammonia as one of the compounds. Now we all know ammonia smells like concentrated urine that has been left to fester for several weeks, so inside the lab was this sinus-destroying stench of urine as we were testing for gas. Out of nowhere, the lab filled with the smell of sewage, and everyone started staggering about in dismay.

'T'cher, why the gas test so smelly one?'
"I don't know, its not supposed to be this smelly =/"

Eventually it got so bad this guy ran over to the window and threw it open, and got promptly knocked back by a solid wall of stench. He couldn't have reeled back further if he had been hit by a hadoken from Muhammad Ali. Turns out there was this big honkin' sewage truck parked right below our lab; so inside our lab was this acidic smell of urine, and outside was a soul-darkening stench of crap. It was so bad even the roaches died.