In 1982 a Hawk at Bedford was undertaking wake turbulence research when it experienced an uncommanded and very low roll;- 20ft. The pilot went out just fractional within the seat survival envelope, while the observer decided to stay in. The aircraft landed upside down and slid 1200ft down the runway. As the structure deformed and was worn away, the observer head was progressively held against the runway. The wreck came to a holt just as the Asphalt was wearing through the fabric next to the chaps scalp.
https://aviation-safety.net/wikibase/55500