A flash of accidental momentary stupidity, forgetfulness, arrogance or ignorance. Normally meant as an attack against Yanks (Americans) aswell. Sometimes refered to as a "Yankee Moment". It is often refered to as a Canadian Slang term of a "Blonde Moment", except a Blonde Moment only means stupidity, not rudeness.
Jim: "Did you hear about the protest in Toronto?"
John: "....Pardon me? Sorry, I was having a Yank Moment."