How can a simple task be so complicated?

I am generally quite capable at working out how to do basic tasks and can pick up new tasks relatively quickly but every now and then one gets you where no matter how hard you try - it just doesn't happen.

Like refilling a franking machine.

My boss decided she would show me how to do it so that she no longers needs to bother about it. I spend most of the postage anyway by the volume of post I send out so I may as well be responsible for making sure there's always money in the machine.

Franking machines are not an unknown item to me so how hard could it be.

Sign in - check
Press the money symbol - check
Enter the amount to refill by - check
Enter the 4 digit security code - check
Done

OK. So, how come it took me and my boss 10 minutes of button pressing, a lot of hysterical laughter and a result whereby we finally completed the transaction only to find we had topped it up with the grand sum of Zero?

Be careful world, we're poised to take over!