One of our club members had an unfortunate experience with a brand new ElectriFly 1250 LiPo pack purchased through a local hobby shop. He charged it through the charge connector and all was well. Then he put it in his plane, a small 4 motored B-17 from a Gillow kit . . . smoke city! 4 motors came on full force in reverse, 20 Amp speed controller toast.
The discharged connectors were wired in reverse at the pack terminals. Deans Ultra that came on pack OK, red to --, black to | so he didn't suspect anything.
Probably an isolated instance but check those new packs with your voltmeter as soon as you get them.