This was a few years ago, I was a new CFI at a new airfield in a plane I'd only flown 2 or 3 times before with the Chief Pilot.
I was teaching the students a takeoff at a very busy airfield (Busiest single runway airport in the U.S. last time I checked).
"We use the rudder pedals to align ourself with the centerline, when we reach .. " etc etc through rotate. At this point I realized the normally very congested freq was silent...
Me: (release PTT)
ATC: Thank you for the wonderful takeoff lesson Nxxx
Me: You'll receive my bill at the end of the month. Please pay cash.
They still make fun of me for that one.