Pitot Flooding
I had a wierd one (to me anyway) a few weeks ago when, after being parked at a slightly 'unusual attitude' overnight in pouring rain, and after 1 normal short flight, the ASI and altimeters both failed at about 300' after take-off.
After the initial disbelief this resulted in a curt call back to the field - then blanking everything around me out and concentrating on doing a circuit pretty much by eye and tacho.
It was uneventful, although being fairly new to type and at an unfamiliar airifield didn't help.
One of the FI's at the club just sucked (gently!) on the static vent, spat out a lot of water, and assured me all would be OK - which it was.
Afterwards I just felt annoyed that this type of flying hadn't been covered in training...