For everyone going “what’s the worst that can happen?”…
I genuinely suspect the chances of getting blown up or zapped are pretty damn slight.
The chances of cocking your car up are very high on some models – I had a Peugeot with multiplexed electrics, and the proper procedure for changing the battery runs something along these lines:
Get in the car, wind down the driver’s window.
Take the keys out.
Get out and lock the car.
Wait a minute.
Pull the bonnet release through the open window.
Disconnect the battery
Reconnecting the battery involves more faffing through the drivers window turning things on and off before and after you reconnect.
Not doing the above causes a computer somewhere to FUBAR itself. The computer is a) expensive and b) frenchist dealer only, and therefore takes weeks to come into stock.
Obviously such unusual and complex procedures with expensive consequences don’t make it worthwhile sticking a warning label next to the battery.
So that’s probably the worst that can happen. 👿
If someone else fits it, at least that kind of aggro is their problem. I found out about the above due a garage disconnecting the battery during mechanical work and putting the car off the road for several weeks of headscratching and waiting for parts…