What was the incident?
Wayne Rooney aimed a swipe at Wigan's James McCarthy with his elbow during Manchester United's 4-0 win at Wigan on Saturday.
Did the referee see it?
Yes. Mark Clattenburg saw the incident and gave a free-kick, but did not feel it merited even a booking.
So why hasn't Rooney been punished retrospectively?
The FA are not allowed to do so under FIFA rules. The FA's disciplinary processes booklet states: "FIFA guidelines aimed at avoiding the 're-refereeing' of matches generally prevent the FA from taking disciplinary action on incidents which are seen and dealt with at the time by the match officials (this includes taking no action). As a general rule, if the match officials see an incident and have jurisdiction to take action, the FA cannot act retrospectively. For instance, these guidelines prevent the FA from upgrading/downgrading yellow and red cards based on retrospective advice provided by the match referee."
What's the thinking behind the rule?
There are several reasons. FIFA do not like to bring in any changes to the game which cannot be recreated at every level, hence their reticence to bring in goal-line technology. They are keen to back the absolute authority of the on-field officials, and also frown upon the idea that referees could go into a game believing they have a "get-out" of trial by video.
Has this happened before?
Often. Aston Villa striker Juan Pablo Angel escaped with a yellow card in 2006 after elbowing Sheffield United captain Chris Morgan, who himself fractured Barnsley striker Iain Hume's skull back in 2008 and faced no further action after being booked; Manchester City midfielder Joey Barton received just a yellow card for stepping on the heel of Portsmouth's Pedro Mendes in 2007, then when playing for Newcastle a few months later got away scot-free after planting his studs in the midriff of Sunderland's Dickson Etuhu in an incident seen but not punished by Martin Atkinson. Nigel de Jong has also benefited twice recently, for his kung-fu challenge on Xabi Alonso in the World Cup final and the tackle which broke Hatem Ben Arfa's leg a few months later. There are plenty more examples.
Have there been any exceptions?
The FA are prepared to act under exceptional circumstances. For example, in 2006 Manchester City defender Ben Thatcher was only booked for an elbow-led challenge on the luckless Mendes which left the Portsmouth player needing oxygen. On reviewing the incident, the FA dished out an eight-match ban, with a further 15 suspended.