This is very embarrassing, but I don't know any of you so I'll tell the story.
I was a flight instructor at a tiny little general aviation airport right out of college in '93. A student of mine had his private pilot checkride scheduled at 9am at an airport about 25miles north of where the flight school was based. We met at the flight school at 7:30, he did the preflight and soon we were rolling down the runway in our C172 on our way to meet the FAA examiner.
On the takeoff roll, we never got an airspeed indication. I aborted the takeoff, had him taxi off the runway and into the runup area. "Must be some ice in the pitot tube" I said as he shut down the engine, "I'll be right back". I got out of the Cessna and very gently blew into the pitot tube. BIG MISTAKE!!
In the 3-4 minns that the pitot heat was on, the pitot tube got very hot, super hot. I was burnt pretty bad, picking up snow and pressing it against my lips. The good news was I cleared the pitot tube and the airspeed indicator worked fine. The bad news was I quickly developed two HUGE blisters, one on each lip.
I had a hard time explaining to people, who know nothing about airplanes or pitot tubes, what I did.
P.S. My student passes his ride with flying colors.