In one of my very first lessons from out of Elstree my instructor spoke to Luton to tell them we would be doing some upper air work in the vicinity, basically over the Hatfield disused. (As it happens Luton's runways are the same orientation as Elstree's.)
Once we had finished, and were back at Elstree my instructor radioed Elstree to say we would be landing on runway 08. Unfortunately he hadn't changed frequency from Luton, and managed to put them in a panic thinking a C150 was about to land, and were busy trying to tell us to keep clear as they had an A320 coming in to land!
I also remember taking my wife for a flight once, and her headset wouldn't work properly: she could hear me, but I couldn't hear her. I told her to use the intercom in the plane because I would hear that.
At that time we were also tuned in to Stansted to keep a listening watch. We were also discussing the irony of the fact that we were tuned in to Stansted given that we were due to fly out of there a couple of days later for a holiday. We were also saying how excited we were about said holiday.
The next thing I heard was Stansted saying, "G-XXXX, just to let you know we can hear every word of your conversation".
Oops!