And you're sure it wasn't super-heated juice from refilling or drawing too hard? How certain are we that it was a "spark" when the entire process was enclosed?
I've experienced overheated juice droplets hitting the tongue (I suppose it might feel like an arc for those not used to getting "bit" and the tangy metallic taste of a good bite) from refilled premiums on high-volts. Just one more reason I've contended from the start that premiums don't belong on high-volt batteries/PTs/etc.
Electrically that's a LONG path to ground unless you're sticking your tongue in there and standing in water.
Consider a 9v battery that delivers about the same wattage. You have to press the battery hard against your tongue to get bit. It can't, even under ideal circumstance, jump an arc through the open air. The open air is a crummy conductor.
Then you consider a silicon soft-cap or a silicon o-ring which is an incredible electrical insulator - or the electrically inert plastic cap and then add the electrically isolated vinyl label on the cart. You consider these things and compare them to a hot droplet of superheated juice and the answer seems pretty apparent to me. [shrug]