I actually thought this had been fixed, but a friend of mine did it in school today. I think he was on a normal blizzard server.
Whenever you want to cap a flag easily, send a rogue to stealth while in the base.
When he gets close to the flag start to summon him with a warlock and when his screen pops up he clicks the flag and then accepts the summon.
There you go, an easy capture of the enemy's flag.
MFG Hydrox
+Karma if it works would be nice ;P