You’d think with the number of years we’ve been sharing files across networks we’d be able to do it somewhat reliably.
Windows: Try to connect to a computer that’s down or misconfigured, and sit for at least 30 seconds, unable to use an explorer window, click on your desktop, or, if you’re really unlucky, use the taskbar or Start menu, until it realizes it can’t connect. (I don’t know if this has been fixed in Windows XP, but it’s still a problem in Windows 2000.)
Unix: On one hand processes are separated better so you don’t normally get a full system lock-up unless it’s trying to connect while starting up…but if you have a modern GNOME desktop, and you have a file in your recent documents list on an auto-mounted NFS share that isn’t available anymore (say, because you turned the computer off), it can lock up your desktop while it tries to connect to create a thumbnail. (This happened to me last night.) And don’t get me started with trying to disconnect from an NFS share that isn’t available.
Mac: Have you got a folder on a server with lots and lots of files in it? Especially images? Hope you can wait for it to transfer every single image over the network and create a thumbnail, because you aren’t going to be able to see anything in that Finder window until it does. (To be fair, I’m basing this on connecting to a Linux box via Netatalk, which implements Mac file sharing. For all I know, connecting to an actual Mac would pull thumbnails out of the images’ resource forks or something.)
Hmm, now that I think about it, generating thumbnails of files on network shares seems to be a problem in itself.