Last night in ICC10 not one, not two, but three iLevel 264 BOE epics dropped...
Ring of Rotting Sinew from a Deathspeaker High Priest, 2% chance.
Leggings of Dubious Charms from a Val'kyr Herald, 3% chance.
Rowan's Rifle of Silver Bullets from Stinky, 5% chance.
For a combined chance of .00003% or 1 in 33333 according to wowhead's drop rates. Should be pretty accurate given the few thousand data points for each mob/item.
33333 raids is 641 years of weekly raids!
Considering we run three and a half hours a week that would be over 13 years of solid raiding if it were possible to repeat the instance back to back.
And after all that can you believe that the shirt from Precious didn't drop once yet with a 30% drop rate? Come on. :)
Not likely to see that happen again in any case.