Pilots make mistakes, and we often try to explain ourselves, or cover up. Some of the excuses aren't even excuses anymore, just a code that everyone seems to accept. Like "the sun was in my eyes" on a missed tennis return (this one is used to explain a poor landing, too). Most of the excuses are things we say to ATC, or that ATC says to us, to help us save face.
Aircraft in busy airspace have to be equipped with a transponder, a device that responds to radar pulses (called "interrogation") by sending out a little signal reporting its altitude and assigned code. The code is mapped in the controller's database to the aircraft call sign, type, and filed destination, so that the controller can see that on her screen, along with the speed and position the radar displays. After take-off, the controller is watching to see that the transponder is responding correctly to interrogation, so she can declare the aircraft radar identified. The old sort of transponder has to be manually turned to the ALT position in order to be ready to respond to interrogation. I am privileged enough to have one that turns on automatically, either squat switch or speed linked, I'm not sure which, but I still hit the ALT button at line up in that airplane, because I don't want to get of the habit for another airplane that lacks the luxury. If I forget, ATC usually acts as though the fault is the transponder's and ask me to "recycle" it--turn it off and on again. That does sometimes need to be done, I'm pretty sure they know that most of the time they don't get a return at the expected altitude, it's because the pilot forgot. Pilots use the same lie. If ATC says, "I'm not getting a return on your transponder," the pilot replies, "I'll recycle" and then reaches over and turns on the forgotten switch.
Similarly if a pilot is a little bit off the assigned altitude, the controller may advise them of the altimeter setting and ask them to confirm their altitude. That gives the pilot a chance to pretend that they were off because they had the wrong altimeter setting. Or if a pilot is told she is at the wrong altitude, she may ask to confirm the altimeter setting. All of these things are sometimes true excuses for the controller not seeing what they want on their scope, but not as often as they are heard.
It was in this context that I heard the lamest excuse ever.
ATC: I see you at 15,400'. Descend to 15,000' immediately.
Pilot: Sorry, my altimeter got stuck.
Might as well say, "Sorry, descending," and leave it at that.