In my works car park of about 80 cars, there are at least 5 cars (Two BMWs, A Volvo, a Jag and a Merc) with parking sensors with very definite bumps where they have reversed into a pole or bollard which must have been positioned between the sensor coverage. I also counted 4 cars without sensors with similar dents.

I think a parking sensor gives the driver an unwarranted comfort factor so they rely on the beeps rather than looking behind.