Your team has a third-and-15 at the opponent's 30 yard line. The quarterback drops back to pass, but a blitz collapses the pocket around him. He escapes towards the sideline and appears to be trying to pick up a couple of yards for the field goal kicker. Two strides before the line of scrimmage, he notices an open receiver 20 yards down the field. With his final stride before the line, he fires a strike to pick up the first down. The stadium erupts, but the cheers turn to boos as the referee turns on his microphone.
WHAT IS THIS CRAP?
This one is based on another rule that helps keep things orderly and gives defenses a fair shot at things.
With one exception, on any given pass play only eligible receivers can cross the line of scrimmage before either A) a pass crosses the line, or B) a defender touches the ball. The exception is on a forward pass that doesn't cross the line of scrimmage (like a screen or shovel pass). On such a play, the linemen can cross the line and be on their merry ways.
If you remember from yesterday's illegal formation post, anyone numbered between 50 and 79 on offense is always ineligible as a receiver. This is true for offensive linemen and for fun times when you like up a defensive linemen as a blocker like Alabama did with Terrence Cody last year. That means that college football teams can't run "tackle eligible" plays like NFL teams can.
Also if you remember from the illegal formation post, the rules are picky about those who line up right on the line of scrimmage. Only those guys who are on the ends of the lines can be eligible receivers. Take this formation:
The part that isn't greyed out highlights the players who are on the line. Only X and Z are eligible receivers, because they are on the ends of the line. The tight end Y is not an eligible receiver because Z is said to be covering him up. He can only pass block in this situation. If he took a couple steps back, then he wouldn't be on the line and could receive a pass. Alternatively, Z could take a couple steps back to "uncover" Y and therefore allow him to catch a pass.
In the scenario in the setup, an offensive lineman probably thought the quarterback was just going to run the ball and crossed the line to run block for him. Had the quarterback just pulled it down and run, there would have been no penalty because there wouldn't have been a forward pass that crossed the line of scrimmage. Instead, the pass made that lineman an ineligible receiver downfield.
If you remember the A-11 offense, it was an attempt to circumvent this rule (as well as the illegal formation rule). You can have fewer than five guys numbered 50-79 on a scrimmage kick formation (i.e. on punts and kicks), a nod to the past when sometimes backs would be long snappers. That way, they wouldn't have to change jerseys during the game. The A-11 exploits the loophole by running every play from a scrimmage kick formation. You can then line up no one numbered 50-79 if you wish, which means everyone could be an eligible receiver. Only five guys can go out for a pass like normal, but there's no way to know before the snap which ones will be going out and which ones won't.
The NCAA rule book requires scrimmage kick formations to be used only when it's "obvious" that a kick is going to happen, so that kills the A-11's viability as an every-down system in college. You couldn't do a deep pass from it anyway, since throwing it "high and deep" in a similar manner to a kick from the scrimmage kick formation is outlawed. It would be possible to run A-11 plays on fourth down, though, as long as it's not a Hail Mary.