255.255.255.1 instead of 255.255.255.0. I hadn't noticed any problems before. But every time I tried to change it to 255.255.255.0, it would go back to 255.255.255.1 when I hit the Apply button. Everything seems to work still, except I couldn't log into my .1 router even though I can ping it.
To my surprise, when I changed the netmask to 255.255.254.0, the Mac accepted the change. And now when I changed it to the correct 255.255.255.0 netmask, it stayed!
This was one of the more bizarre network strangeness I've seen. Hopefully, the hint will save some headaches out there. Note: I'm running OS X Server 10.2.6. Don't know if the anomaly has been seen elsewhere.
[robg adds: If anyone has any thoughts on what might cause this problem, please post them!]

