Wow, that's impressive. I ferment 2.5 gallons in 3 gallon carboys and haven't had a blowoff in them yet, but this is making me wonder about having 0.5 gallons of clearance... Was it high-gravity?
I made an imperial stout last Friday.. when I came down Saturday it had blown off the cap to the airlock and the little bubbler device.. It also left some on my 8ft high ceiling....