A colleague gave himself a £600 bill many years jump starting a friend with his 1st gen Laguna, blew the dash, no doubt only some cheap parts were blown but increasingly even finding someone capable or willing to fix such units was difficult.

He was an experienced car transporter driver so would have multiple experiences of jump starting across flat batteries, many's the time we'd have to fit two or more saets of leads to start Diesel engines such as Transits and Terranos such was the requirement for current.

Had a heart stopping moment a few years ago in my own works car park, freezing morning and a Corsa wouldn't start just a dead thump of the starter, jumped it off my Landcruiser (twin batteries so massive CCA) still only a thump, i'd let him connect his side so wondered if a nasty result for me, however the penny had by then dropped and after i asked him if his car had any anti freeze in and his reply being no, i removed my leads wished him luck and went home.

HJ used to warn against jump starting Toyota hybrids, stated damage would be revealed later on, its not a system i'd want to risk jumping.

I have a proper heavy duty jump pack with master switch, can get all conections safely and double checked before switching on.

You are right I have jumped loads of cars, but made the mistake once on the job ( recovery ) of connecting the leads up the worng way and breaking the alternator, it is the operator at fault not the action of jump starting a car.

I always get the battery checked if it was not because I left summit on in the car.