Been slowly scrolling though the Wikipedia list of airshow crashes and incidents.

Lots of low-level failures to account for altitude conditions, wind, and I think partially just old planes failing.

This one I think was the most surprising so far regarding cause:

September 11 - (Mannheim, Germany) - A United States Army CH-47 Chinook (serial number 74-22292) crashed while carrying British, French, and German parachutists who planned to jump when the helicopter reached an altitude of 12,000 ft (3,700 m). All 46 aboard were killed. The crash was later found to be caused by an accumulation of ground walnut shells that had been used to clean the machinery.