What happens if you connected the battery up wrong when giving a jump?

Any one or more of the following:

-nothing

-battery explosion (and resulting damage possibly including permanent injury)

-fire

-blown fuses

-burned wiring

-damaged alternator or AC compressor

-damaged electronics including such things as radio, engine computer, body computer, gages, ignition module, transmission computer, antitheft computer, door latches, airbags, etc.