I always thought an entire section of the cockpit is blown off upon hitting the ejector.
It used to be like that, but it isn't anymore.
There were a few accidents where the ejecting aircrew hit the opening canopy/cockpit cover, causing serious injury and even death.
- A particularly nasty one was the head of the rear seat occupant hitting the edge of the canopy as it swept up and back and the seat rocketed up to meet it, which could even break their neck.
So in modern aircraft, the top of the cockpit is shredded into tiny bits by an overkill set of explosives before either occupant's seat fires the rocket, so there isn't anything big enough left to do much harm.
- You might want to close your eyes though, as you may well be going through a cloud of glass if speeds are low enough. Sure beats going through a sheet of bullet-proof glass though!
Helicopter ejection systems are even more fun, as they have to blow the rotors off before ejecting.