Some cars will zap the radio when the battery is disconnected. Honda for sure.
Need a dealer code to get it to work again.
more:
Tool I want...but don't know the name? - The Garage Journal Board
Battery Disconnect Problems
Toyota (various models) If the battery is disconnected while the key is on, it can set a fault code for the airbag system and turn on the airbag warning light (which deactivates the airbag system until the fault is cleared with a scan tool).