They hit overhead stuff pretty often.
Bed up alarms are options, frequently broken, disabled, or ignored. Depending on the components of the truck speed limiters can be designed and installed, they are far from 100% reliable. PTO governors are usually pretty reliable, however on trucks used for snow removal and deicing they are usually not set, and old trucks will have none of this stuff, just mirrors which should be enough.
Pretty sure taking off without lowering after dumping is just as common as the bed accidentally getting raised. As they move from mechanical/pneumatic valves to electric over hydraulic the incidences will likely decline along with reliability.