Not just mobiles...
I run a small business from home, and the other day I was paying one of our suppliers (using a PC with 24 inch monitor), and instead of typing 2585.10 in the payments field, I typed 25851.00. Easy mistake to make, and I was a fraction of a second from pressing the "confirm" button.
What if I had? Why is there no "undo" button? Yes, I know it asks twice to confirm, but does anyone actually think about it - aka. sending a snotty email to "reply to all"? It only takes the "ohnosecond" to realise your mistake, so there really should be an undo/cancel button, that will work for say, 30 second after confirming a transaction.
What do you do if you make this mistake - phone the helpline and be hanging on for ages and talk to an offshore call centre, explaining the predicament? In my case it was a small supplier so I could have rung them and I'd like to think they would transfer it back, but imagine trying to get someone like BT to refund the excess amount. It could take weeks or months, which for a significant amount could bring down a small business.