It can F up for so many reasons:
Crappy cable
Weak USB port
Bad/loose connection
Flat battery
Incorrect software versions and or drivers combo...
Inattention.
I had a hot reflash fail recently, deep breath, swapped USB ports to the port closest to the ext power plug on the eeePC*, test reflash, reflash, success, relief!!!
*Also try the shortest fattest newest (least munted) USB cable possible...
Not too short that you'll rip it out by accident mid reflash...
Last time it happened before that, I panicked, keyed off and bricked.
Let's say experience count$$$
