A discussion arose last night about how to determine when Leap Year was. It was pointed out that whenever it is an election year, February also has 29 days. This is generally true, I pointed out, except in several rare cases, when every fourth year is NOT actually a leap year. People scoffed, said "Silly Jessie, doesn't know what she's talking about..."
Well, turns out I did know what I was talking about. According to Wikipedia, "The Gregorian calendar, the current standard calendar in most of the world, adds a 29th day to February in all years evenly divisible by 4, except for centennial years (those ending in -00), which receive the extra day only if they are evenly divisible by 400. Thus 1600, 2000 and 2400 are leap years but 1700, 1800, 1900 and 2100 are not." [Emphasis mine].
It's not often, but it happens.