The pilot involved in the Lowestoft display Crash only suffered minor injuries sustained from........ landing on the wreckage!
As to the UAS girly several years ago. She was indeed too light for the ejection seat and was unlucky enough to land back in the wreckage. The pilot, who landed close by, re-entered the burning wreckage to drag her out, both of them thereby sustaining burns.
Outcome of being too light for the ejection seat may mean that you do not clear the aircraft to the degree tested. Conversely, you can also be too heavy (pushing the aircraft away rather than being fired clear
don't take this last bit too seriously)