I choose "If you leave your mobile phone at home..."
If you leave your mobile phone at home, everyone will call you or send you a one thousand messages, and half of this mails are really important for you.
I hate this momemnt when you forget your phone, and you need it so much. Just imagen situation when you or somebody else get lost, or there is an accident, or now you're going to have a test and all important information is in your phone.