At the heart of this play is football's rule concerning receiver eligibility—in general, lineman and tackles (who generally wear uniform numbers between 50 and 79)—are ineligible to receive forward passes or run downfield to receive such a pass. Detroit's Decker wears #68 and is thus ineligible...that is unless he reports to the referee to declare himself eligible for a specific play.
Replays indicate Decker approaching referee Allen prior to the play, but so did two other Lions players, including offensive lineman Dan Skipper, who wears #70.
Pursuant to football officiating mechanics, Allen quickly ran to tell the defense and made a public address announcement, twice stating that #70 had reported as an eligible receiver for this play.
Detroit ran its play and #68 Decker caught quarterback Jared Goff's forward pass in the end-zone for an apparent successful two-point conversion...until Allen announced a penalty for illegal touching, reverting the score to Dallas 20, Detroit 19 (a subsequent two-point conversion attempt failed and Dallas won the game).
At the heart of the matter is the confusion and miscommunication that occurred when both #68 and 70 approached referee Allen, who dashed off to tell the defense and announce the eligible reporter, apparently reading "70" when Detroit meant "68"...except Detroit either didn't notice or otherwise didn't correct the referee when the public announcement over the PA sounded as "number 70 is eligible."
Lindsay's Call: I am not a football official but of the sports I do officiate, it would appear to me the primary culprit may have been a player stating "I am eligible" (or "I'm reporting" or some other variation) and the referee assuming that because #70 had reported eligible earlier in the game, #70 was reporting again.
To remedy this conflict, might I suggest white hats opt not to take the pronoun "I" as sufficient identification, especially when multiple players who may all declare eligibility approach the referee at roughly the same time? Instead, I would suggest the referee read back a number (or outright ask "What number?") in order to confirm the identity of the eligible player reporting.
That way—through stating the number of the player—this communication loophole can be closed.
Video as follows:
