http://ohsaafb.com/files/2015/08/201...ifications.pdf
Page 38, *5.1.1 SITUATION A:
What procedure is used to correct an
error if it is discovered: (a) a fifth
down has been run; or (b) A has not
been given its allotted four downs?
RULING: In both cases, the number
of the next down can be corrected by
the referee until prior to the ball
becoming live after a new series of
downs is awarded. In (a), play will
revert to where it was before the fifth
down was run if it was determined
before prior to the ball becoming live
after a new series of downs was
awarded. In (b), the referee can
declare the proper next down provided
prior to the ball becoming live after
a new series of downs has not been
awarded. …