What are the possible ways you could go over your mobile contract?

I have an iPhone 4S and I'm on vodafone. My bill is meant to be £17 a month but I checked my allowance and it says it's £18, how could I have gone over it? What are all the possible ways? I would like to know so I could hopefully prevent it for the future.

Sending picture messages if it's not in your package, sending too many texts if it doesn't include unlimited, using too many minutes and using too much of your Internet allowance.

Tip: turn off mobile data where possible and do all Internet stuff (including email) over Wi-Fi. This does not use up your data allowance. If you keep mobile data switched on your phone automatically downloads emails and updates open pages like Facebook, this can eat up data that you don't know you're using!