For me, this is a result of human error, accidents are known to happen and will continue to do so.
If a player wins a penalty by diving and probably gets the opposition player booked, if not sent off in the process, do we call for the team to allow a 'walk-through' goal or to play with a man down in return?!... We can all see that refs make these mistakes almost weekly but teams don't get goals rescinded or matches replayed even though they didn't deserve to concede.
Watford were unlucky, but this was an honest mistake - nobody cheated - sometimes crazy things happen.
ie - the wrong Aalborg player got sent off just last week, there was that goal scored by the ballboy, Simunic getting 3 yellows in the World Cup match, Maradona's 'hand of God' and many many more examples have happened over the years including loads of goals scored off the ref.