This troper recently heard a story that embodies this trope; a young man kissed his GF goodbye, put his helmet on and rode off on his motorbike. While still in sight of the GF (so not travelling that fast) he skidded out and came off the bike. He smashed, head first, into a lamp post, causing his helmet to pop off...containing his head...A subsequent investigation showed, ironically enough, that had the lad not been wearing a helmet, he'd have shattered his jaw and most of his face but would likely have lived. It was the 'lip' effect of the extra inch or so of helmet on the lamp post, that caused his head to be ripped from his shoulders.
DISCLAIMER; THIS IS NOT IN ANY WAY ENCOURAGING PEOPLE TO GO WITHOUT HELMETS. THIS WAS A ONE OFF FREAK ACCIDENT AND HE COULD STILL HAVE DIED WITHOUT HIS HELMET. HELMETS SAVE LIVES. Still, turns out we're more fragile than we realise.