I've seen it happen a few times before in lower league games and never really thought too much about it but there's just been a tannoy announcement at Wembley asking if there's a qualified ref in attendance to take over as the 4th official following an injury to one of the officials. Surely you can't have a fan of one of the two sides officiating, even just as a 4th official, a Premier League game? With all the billions of pounds in the PL surely they can afford to have a 5th official on stand by for situations like this.