For example:
Lincoln had a secretary called Kennedy, Kennedy had a secretary called Lincoln.
This is one of those coincidences that isn't a coincidence at all -- it's simply wrong. John Kennedy did have a secretary named Evelyn Lincoln, but one searches in vain to find a Lincoln secretary named Kennedy. (Lincoln's White House secretaries were John G. Nicolay and John Hay.)
Taken from:
Urban Legends Reference Pages: History (Linkin' Kennedy) which dismantles this urban myth quite thoroughly.
Apparently this list of coincidences first originated around 1964, so it's even older than I thought.