Robert F. Kennedy was assassinated June 5th, 1968. Mad had a cover ready to go featuring Alfred, pin in hand, ready to burst the balloons of the 1968 presidential candidates pictured below. RFK was to be featured, but the assassination forced Mad to insert Alfred into RFK's place.
Mad #122 published front cover
Mad #122 altered by Doug
One other note: This is one of the few Mad covers illustrated by two artists, Norman Mingo and Mort Drucker.
Before the events of September 11, 2001, Mad had planned a completely different cover for issue #411. Thanks to Mike Slaubaugh and Dave Williams for the information.