Someone really does not want Manchester City supporters to go to Wembley. According to the Greater Manchester Police, 900 tickets (out of an allocation believed to be around 30,000) destined for the Eastlands were stolen from a special delivery bag in a Royal Mail van on Friday, leaving several hundred City fans having to do without. A Royal Mail worked has been arrested on suspicion of theft, but it's currently not clear whether the suspect is a Manchester United fan.
A statement on the Manchester City website seems to indicate that there won't be any problems related to the theft, however:
All supporters impacted by the theft will be contacted by the club in the coming days. Supporters are strongly advised not to purchase tickets from unofficial sources. Only tickets purchased direct from the club will be valid for the game.
It doesn't seem as though it would be particularly difficult to cancel the 900 tickets stolen and reissue new ones since they all should be tracked and earmarked for individual fans if they're being sold through the clubs. However, I'd guess that more than a few fans will travel down to London on scalped tickets and not be able to get into the game, which is pretty unfortunate - and that's if the organisers don't manage to mess up the changed tickets in the first place.