### Clue: Error from Messi's header and team's back in the lead (3-2)
### Answer: MIX-UP 
 ### Explanation: Definition: Error
 **1st letter of**(…'s header) "Messi" **plus**(and) **reversal of**(…'s back) XI(Roman numeral for 11, used to refer to sporting teams with that number of players) + UP(in the lead in a competition, as in "the Rovers XI were 2 goals up on their opponents").
Sounds like Lionel Messi made a boo-boo with his header.