Bent conrod
Somewhere in my office there is a very bent conrod, out of a C310 engine. This engine was primed before the starter was engaged, and the prop only turned about 15 degrees and stopped.
The engine was wrecked. Conrod, piston, crankshaft, crankcase-all unuseable.
One cylinder had been full of fuel.
This engine had a problem with the fuel control unit, but there was also a note in the POH saying not to prime unless the propeller was turning. This applies to Cessna 340's and some C310's.
Read the book!!!