Also known as a running gag
should be a joke
that was kind of funny
the first time, but one that gets progressively worse
as it's repeated. Eventually, you will achieve a break-point
where the joke magically becomes funnier
than the first instance when it was told. At this point, caution should be exercised, because the joke will rapidly deteriorate.
IMHO The Muppet Show
used to have the best running gags.