To be fair this is how Ad-Hoc networking works; anyone is free to create a network and the OS will allocate an IP from a private range "commonly used" for such things (ditto when connecting to a network and not receiving DHCP reply), then the machines can use Bonjour/Zeroconf to discover each others services and start talking.

It can be a bit confusing though if the OS remembers ad-hoc connections it has sometime connected to (vs created), OSX for example doesn't do it (but if my N900 goes down while my macbook is connected to it, it will [as per spec] keep the ad-hoc network running on it's side [possibly losing the N900 DHCP assigned IP at some point).