Well, a rumor is that the bug was allowing more people the be in an instance or raid than should be. Apparently, the check was made at the time of the summon, not the acceptance.
Therefore, if 4 people went into an instance. Warlock summons the stone, and 2 summons were started at the same time, you could have 6 people in a 5 man.
In a 10 man, bring 9 in, summon stone, start 4 summons and have 13 people in a 10 man raid,
In a 25 man, have 24 people in, 12 summons, 36 people in a 25 man raid.
Definately broken.