I use our 3GB Data Plan VERIZON MIFI unit alot when on the road and we got bombed one trip with a bill for going over the account limit. Turned out it was the downloading of MS UPDATEs on our laptops. Since we always usually have both laptops up and going I probably got multiple billing for the same thing haha... We had to set our laptop for manual downloads after that to make sure it didn't happen again...
Always found VERIZON helpful folks to deal with... They didn't back off on this particular deal though however haha... It wasn't as bad as what happened to you however...
Right now VERIZON is starting a big campaign to make all of us grandfathered users of their data plans to start paying more for the data used to be inline with all of the other internet providers.
Roy Ken