RE: Here's a new one....
I've had issues with the plates that the power wires are soldered to delaminating, and causing an intermittent connection. THe wierd thing is it appeared to have delaminated between the base metal of the contact and the solder, which mean it could be stuck back on again.
Thankfully this was for an RC, so I didn't get too upset, and the pack had had a haaard life.