"Following a technical problem with the software of an external service provider that instructs the officials as to which teams are eligible to play each other, a material error occurred in the draw for the UEFA Champions League Round of 16. As a result of this, the draw has been declared void and will be entirely redone at 1500 CET", UEFA tweeted.
According to reports, the software that was supposed to keep track of which teams cannot be paired, glitched. As a result, Manchester United and Villarreal were matched together, despite them already having played against each other in the group stage.