I had to jump start someone off last night, they hooked their side up wrong (crossed), my JK and his land cruiser seems to be ok, but im still very worried about the electrical system/battery/alternator etc.

If no problems have showed up immediately, am I ok, or could things crop up later down the road?

 

First of all , welcome to the forum. I think if anything bad happened it would be noticed immediately

 

I highly doubt anything happened on your end. Seeing his battery was already dead, there's no way for enough power to be shot your way. Next time only trust yourself when putting the cables on.

 

Get some White-out (easy to see day or night) and mark your batteries, that makes it idiot-proof.

 

you would know by now if there were any problems. I'm amazed people do that though!