This is a shock that doesn't travel through cables or wires. This wireless shock mostly arrives on paper or through email. It's called the ‘mobile bill' shock, also lately known as the 3G shock. It jolts you only when you ‘touch' mobile plans without first analysing them.
So, the first safety rule is, check the plan. If you are on 3G and subscribe to a 250 MB plan, you are inviting trouble. You will not realise that the ‘free' download has evaporated within days as you continue to download videos and music clips at ‘3G speeds'. Only after the bill arrives do you realise you have downloaded 2,000 MB!
This happens because after the ‘free' limit, you will be charged 10 paise for 10 kb; that is, a paise for every kb you download. Just convert the MBs you have downloaded into kbs, and you may be in for a killer shock.
Telecom companies tell you how many MBs you are entitled to download free. . But after that, the rate is in kb, not MB. The onus is on you to understand the finer details.
This practice is, in fact, common even among vegetable vendors. If you are told onion costs Rs 5, it is up to you to ask whether it is for a kilo or 250 grams. The vendor is not lying. But if you assume that it costs Rs 5 for a kilo, when it is actually for 250 grams, you are cheating yourself, or are being plain stupid.
If you can't tell kb from MB, ask the telecom guy. Many service providers will readily tell you what your average bill will be like if you use specific amounts of data.
It is also better to insist on a daily download report from your provider. Or at least, ask whether you can track your usage online.