It's more rumour than proof. But, it's said a bank worked out that an X25 call reject response had enough data space to pass a transaction payload and coded their ATM to use incomplete calls to pass the higher protocol messages. Because X25 was modelling phone networks it only charged for completed connections: it was a free call.

The protocol got a new minimal, mini-call model which didn't expect more than one packet each way to take account of it.