Here in central IL I believe it was two years ago there was a plane they used for jumping. After everyone jumped it looked as the plane stalled and one wing dropped. They believe there was such a force when the plane stalled the pilot hit his head and was knocked out. Not sure if that is what happened.
Are you referring to the crash near Taylorville (I think it was Taylorville....)? Given that there were witnesses who said the aircraft was maneuvering to avoid the buildings, I don't think that is probably the case. However, the chances of knocking one's head hard enough to knock oneself out is a real one in a lot of the "faux fighter" type homebuilts because of tight fit necessary to maintain the desire aestetics. Even in non-fatal crashes, it's not uncommon to find blood, hair or even pieces of skin stuck in the frames on the sides of the canopy or on the instrument panel. Somewhere I have a picture of a "faceprint" from the face of a RV pilot being bounced off the side of his canopy during a crash. He survived (thankfully there was not a post-crash fire) but had to have his face bolted back together by a maxillofacial surgeon.

I would not fit in my airplane with a helmet so I modified my airplane by lowering my seat.
I did the same thing but I am working on my design around a 6'3" 230 lb occupant. If someone that size can fit in there, my 5'9" 170-something pound frame should be able to easily accommodate a helmet with some room to spare.