I've tried it in both glider and power
.
With the glider it was when I had left it out under covers without sealing the pitot properly. The water that had leaked into the pitot system in the rain was enough to give some very funny readings on the ASI (winch launch so didn't abort) with the speed jumping about in 10kt steps. I flew on the hill for an hour or so just ignoring the ASI and then did a slightly faster than normal approach and landing without any great problem. Now I'm very careful to block off the pitot when it's left out under covers. I also tape over the upper statics in any showers when on the ground waiting to fly as they are on top of the fuz and vunerable to water ingress. I find a bit of grass under the white tape over the static is enough of a reminder.
On the power side it was with our AA5 in the winter as a result of ice in the static vents. The plane is normally normally parked outside under covers. With a vent on each side of the fuz you wouldn't expect it was possible for both to become blocked with ice. Nothing untoward on preflight check, taxi or take off but the problem became obvious in the climb as I had to lower the nose to maintain airspeed until I just stopped believing it. The altimeter stopped rising in the climb too.
There's no alternate static in the AA5 so I just ignored ASI and altimeter and flew by power and attitude. A quick return to the field and a bit of heat on the static vents soon sorted it out. Nice VFR conditions so no need for anything as extreme as breaking the VSI glass.
Alastair