Once I was changing termal paste on my Windows based expensive tablet PC... And I was so scared to disconnect any cables (there was many of them), so I did it all with battery connected (I didn't even knew where is battery cable). When I tried to put board shield back... it didn't go right into needed place... and short circuit some small component. It flashed. That was the end of my repair. Dudies from repair service later told me that multiple components fried including CPU, so repairing is too hard. That's what can happen if you don't disconnect battery.