When trying to jump start my wife’s car, there was a big spark when I connected the cable. Unfortunately, some idiot had labelled the battery connectors on my wifes’s battery incorrectly…
A few items stopped working on the Daimler, but thankfully it was just blown fuses.
Back to normal now. Could have been a lot worse.
Exactly … I have heard horror stories about that sort of thing involving our Jags … 
Sacrifice a little something to the Jaguar gods some time soon in appreciation … 
alternator diodes hate that, so keep an eye on that
dont ask me how I know
dont ask me how I know that if, in the dark, you connect a dead flat battery to a older style charger, it will charge up with reverse polarity, and that when later inserted into the car, it will spin the starter motor
In my case, smoke emanated from the alternator
This was a confusing and annoying lesson
Also, don’t ever try to connect up a dead battery to the good one in your car (i.e. in series), hoping the charging system will also charge up the dead battery while the engine is running. 
Good bud of mine (and a former NTB tech, no less) tried doing this trick with his big Chevy SUV one time. Further on down the road he could suddenly smell something burning, followed by a big cloud of some white “smoke”. 
The combo apparently resulted in the dead battery being WAY overcharged (can’t remember whether it damaged the charged battery as well), causing it to rupture and battery acid fluid to run down and onto one of the a.c. hoses which it quickly ate through, causing sudden loss of a.c. refrigerant (I think this is one of those SUVs that also had a.c. fixtures in the rear compartment). 