Whad'ya Know?
Oh Crap, we're a man short!
If you think the USSF referee test is easy, consider this question from the National Referee's test. Thanks to Kevin Stott, a MLS and FIFA referee who posed it to us at the 2004 State Referee's meeting.
When a player re-enters the field without permission, you would normally just blow the whistle, show him a yellow card and restart with an Indirect Free Kick for the opposing team (see Law 12, Fouls & Misconducts, #6 under Cautionable Offenses). Please note that the player is not a substitute. Since his team is playing short-handed he is still considered an active player.
Of course, the opposing team will be pretty upset since the player who re-entered the field illegally denied them an obvious scoring opportunity, although he did not commit a foul. So let's have a further look at Law 12, #5 under Sending-Off Offenses:
5. denies an obvious goal-scoring opportunity to an opponent moving towards the player's goal by an offence punishable by a free kick or a penalty kick;
The key here is "offence punishable by a free kick". It does not say "penal foul" or "foul".
Since re-entering the field without the Referee's permission is punishable by awarding a free kick to the opposing team, the correct ruling here is to show the player a Red Card (the more severe punishment) and award the opposing team an Indirect Free Kick.
This is 100% cut and dry, but most of us undoubtedly have not read this line that carefully. Note that you would do the same thing if the break away had been stopped due to obstruction or dangerous player. If you have a wild imagination you can perhaps devise a scenario involving dissent.
Note that in a room full of Level 5,6 and 7 referees all kinds of answers were suggested!