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.
also i've seen this posted on other sites as of today, so i reckon it still works.