starcraft-2
This is something I've fixed recently and thought I'd document for benefit of others, as it's so obscure.
When anyone on my network downloads a custom StarCraft 2 map, the map download will fail (or appear to fail and later work) and all connections (even on other computers) will be dropped; they'll sign out of battlenet, skype, msn, and pings will time out for a brief moment.
While I'm not sure as to what this problem was caused by, it seems to have been fixed as of Patch 1.1.
Back in Warcraft 3, there were a number of communities dedicate to the play of a very specific custom map: "Defense of the Ancients: Allstars"
While other maps did reach some level of acclaim it was DotA which overwhelmed the entire custom map listing method to the point where no other game could reliably be played.
I took this picture on a DotA light day:
As playing DotA began to tax the in game custom map system splinter communities (eg. Garena) formed.
Sadly, Starcraft 2 hasn't been around long enough to receive a game like DotA and its custom map system is far more efficient than its predecessor. The result is no such communities have yet formed.
My advice might be to wait a while, or attempt to start one yourself.
Best Answer
The solution is to fire up cmd and reduce MTU size;
Where "Local Area Connection" is the device name for the adapter you connect to bnet through.
The problem was the router; Belkin F5D7633-4. Even the latest firmware only allows TCP/IP packets to be ~1492 bytes, despite 1500 being ethernet max. My theory is that the router was fragmenting the packets, and then either this process or the response from battlenet to these router-fragmented packets caused the router to crash.
The MTU value I determined following this process;