Originally Posted by Robert Goodman

Letting my imagination run free, I can pretty easily cook up a scenario to match the enforcement I wrote of above. A pass ended incomplete behind the neutral zone, and just after the ball fell dead a player of B, in a way that was judged gratuitous, slammed into the receiver who'd missed the ball. Referee assesses an unnecessary roughness penalty after the ball became dead, i.e. after it was already 4th down. The choice being obvious, he sees no need to present the option of declining it, and just tacks on 15 yards -- but then he forgets which 15 yard penalty he's given, and signals for unsportsmanlike conduct instead of a personal foul. Because of a muscle pull, he holds his arms at an angle out in front of him instead of straight to the sides while giving this signal.

The line judge, having seen the aftermath of the foul but not its development, because his att'n was elsewhere, sees the referee's signal and thinks it's for pass interference, which is fine with him because the LJ doesn't realize the hit was late rather than early, and because the LJ had gone downfield and lost track of where the neutral zone was. So the LJ tells you it was an interference call. By the time he realizes otherwise, he has other things to do than to tell you what happened.