Can unhooking negative cable cause damage to anything

No damage but definitely a reset on some systems but nothing to worry about...

It will not damage anything because once the cable is removed from the battery terminal, that stops 'ALL' current flow as battery is eliminated from the system until cable is reattached to the negative battery post .. However some of the onboard control modules will loose memory and will need some run time to reset... It may also cause radio security coding to be erased, which requires 'manual' code entry to restore radio functionality.

see Pushrods answer #1....he's right.it won't damage anything as long as it don't touch the positive anywhere!