Re: User Error
Sometime mid seventies, some UK Fords came off the line with imperial heads on metric bolts. (Or possibly the other way around - it's been a long time since I took one to bits). I always assumed it was something to do with their production line transition to metric fixings.
(And I recall a Datsun of the same vintage which was metric throughout, except for a single 3/4 BSP thread for the temperature sensor. Though that had at least some excuse: the engine was a licensed version of the BMC engine found in Minis and Morris Minors.)