Pitot heater
Ahhhhh...
I've had the 'Turn it on all the time because the check list says' comment (which never made sense because it was not backed with an explanation).
I was also told its designed to work in a 100 mile per hour wind and doesn't like being turned on for long times on the ground - which did make sense so turn it on at take off and off when you land.
One instructor once said 'since we are not going to get near 5000' on the trip we will only turn on the heater if the OAT says less than 10degrees'. That seemed to make good sense but then I remember my first comment about the compass.
So now after 700 hours the 'Turn it on all the time argument seems to make sense'. But I can now explain why.