I have Virgin Mobile Prepaid as a cell carrier. When checking my account balance for remaining minutes, I have caught them twice now in shortchanging my remaining minutes.
I suspect they do it every month. For example, just now I found that their account balance says I have used 285 minutes, when in fact I calculated usage at 262 -- and I did this three times to make sure I didn't miss something. If you call their customer service they will fix the discrepancy, but I shouldn't have to do this at all. Imagine how much extra revenue they get in misinforming their customers of their usage, because most people likely do not check their math.
It's easy to do - just highlight the tables and copy them into Microsoft Excel. Then use the simple autosum formula.