When the first Palm VII's came out (https://en.wikipedia.org/wiki/Palm_VII) I wanted to see how responsive the wireless connectivity was before buying one. The local Staples store had a functional demo unit, but it had no subscription. So I tried signing up with random info, when it got to entering a card number, I entered 4111 1111 1111 1111, which passes the basic validation checks for a visa (and is obviously easy to remember). The device activated immediately and I was able to try some live data transfers. It worked for about 3 days (I went back to check it out of curiosity).

Presumably the activation of service happened locally in the device, or with minimal cross-checking with the backend billing service.

A few months later I tried it on another unit and the number no longer worked for activation :)