I use Parallels Desktop for Mac to run the Action Request System (Remedy) for trouble ticket tracking at work. They have a webapp, but it's not really usable.

A couple weeks ago, about when I upgraded my work desktop to Leopard, Parallels broke. I couldn't connect to the Remedy server, or our voicemail system. I don't really think about Parallels networking, but it's all virtual so normal troubleshooting is unavailable. Basically there's a fake DHCP server (or two) inside Parallels for the VMs, and I had very little visibility into why it was doing the wrong thing. I reinstalled Parallels but hadn't spent much time on it, since I don't use Remedy heavily.

It turns out I needed to re-set Parallels from Bridged to Shared networking mode, whereby it uses the Mac as a NAT server. The NAT alleviates many of my concerns about running Windows. But how & why did that setting get changed in the first place??