I once had a boat with incorrectly fitted gear cable. This gave me no reverse gear - but left the engine in forward. As I tried to apply reverse to slow the boat down the opposite happened which made for an interesting moment! Probably not what happened here, but I'm a little less critical of other's 'cock ups' since then.
Oh the memories of being on a 10 berth river cruiser at the end of a heavy drinking weekend coming back into Roys of Wroxham mariner just as the same happened to us. The damage still makes me wince