This is not directly a DNS problem, it's a network routing problem between some parts of the internet and the DNS servers for serverfault.com. Since the nameservers can't be reached the domain stops resolving.
As far as I can tell this is a the routing problem is on the (Global Crossing?) router with IP address 204.245.39.50.
As shown by @radius, packets to ns52 (as used by stackoverflow.com) pass from here to 208.109.115.121 and from there work correctly. However packets to ns22 go instead to 208.109.115.201.
Since those two addresses are both in the same /24 and the corresponding BGP announcement is also for a /24 this shouldn't happen.
I've done traceroutes via my network which ultimately uses MFN Above.net instead of Global Crossing to get to GoDaddy and there's no sign of any routing trickery below the /24 level - both name servers have identical traceroutes from here.
The only times I've ever seen something like this it was broken Cisco Express Forwarding (CEF). This is a hardware level cache used to accelerate packet routing. Unfortunately just occasionally it gets out of sync with the real routing table, and tries to forward packets via the wrong interface. CEF entries can go down to the /32 level even if the underlying routing table entry is for a /24. It's tricky to find these sorts of problems, but once identified they're normally easy to fix.
I've e-mailed GC and also tried to speak to them, but they won't create a ticket for non-customers. If any of you are a customer of GC, please try and report this...
UPDATE at 10:38 UTC As Jeff has noted the problem has now cleared. Traceroutes to both servers mentioned above now go via the 208.109.115.121 next hop.
