Mobile broadband users face stiff penalties for exceeding their download limits even though most aren't aware of what those limits are.
A survey conducted by price comparison website Moneysupermarket found that three in four mobile broadband users risk such charges.
It found that over half did not know what their limit was, with a further 24% not even realising they had any.
O2 charges £200 for every extra gigabyte used and 3 charges over £100.
By contrast, Vodafone charges £15 per extra gigabyte with Virgin and Orange offering a £14.95 penalty.
Only T-Mobile has no charge and will advise heavy downloaders to change tariff.
Mobile broadband, in which users plug a so-called dongle into their laptop or computer, is becoming more popular.
"Dongle users are most at risk of incurring charges from exceeding download allowances as limits on dongles tend to be much lower than fixed-line packages," said James Parker, manager of mobiles and broadband at Moneysupermarket.
Someone with a three gigabyte limit would be able to do a few hours of surfing per week, download three music tracks per week, watch two hours of video on YouTube or iPlayer, download one film and spend a few hours on Skype.
"You'd only need to double that usage to find yourself with a £600 bill from O2," said Mr Parker.
In its defence, O2 said that the high penalty was "used as a deterrent and to make sure that others using the network had a good experience".
"Very few of our customers go over their limits," said a spokesman.
"We text them when they are at 50% of the usage, and again at 90% and again if they go over. We keep them well-informed and people understand our pricing structure," he added.