I recently received a message from Fido system, saying that I have exceeded my monthly limit of data (4GB). When I looked into it, turned out that the system registered a transfer of approximately 3.5GB in a span of 1 minute. The information was confirmed later by a Fido customer service people. At the time reported my phone was tethered to my desktop computer. There was no downloads, Windows or any software updates in the background, no streaming or any other activities that could incur such high data usage. Furthermore, even with the high speed of Fido's mobile internet transferring 3.5GB of data in a few seconds would be impossible. For the reference, in the ideal conditions it would take approximately 10 minutes to transfer a single file of that size over a 80Mbit/s network.
The actions performed during the time my phone was tethered were; getting a few emails (no attachments), browsing 2-3 web sites (no streaming video) and one more operation, which seems to trigger an error in the data counter used by Fido:
I removed a folder from my Dropbox on the computer, containing a lot of JPEGs, the total size amounting to approximately 3.5GB. Now, the way any cloud service works, when the files are deleted locally, the computer send a list of files to be deleted to the cloud server. The list of files would amount to a few Kb of info at the most. It seems, however, that the data counter on the mobile network decided to bill me for the total size of files removed.
A situation like that is unlikely to happen often on this scale, because the phone would have to be tethered, a cloud service would have to be used and the amount of files removed would have to be very large to notice right away.
However, it is quite possible for the system to constantly make an error on a small scale, without users noticing it and, therefore, being overbilled for the data they have not actually used.
I am currently waiting for an explanation and resolution from "The office of the president" as I was told.