4 in 15 years:
1 in a glider, simply bounced off
1 of the prop tip in a Piper Cub on short final, made a funny noise (twing!) and bounced off
1 at night in an Aztec at an unholy speed in the descent, bounced off the nose and splattered all over the right engine.
1 on the horizontal stabilizer of another light twin on downwind. That was the scary one. It did cause damage.