It's just another example of why the FA are bellends for having the semi-finals at Wembley, but Millwall should also have anticipated the potential danger of flogging an additional 15,000 tickets to what was needed, presumably to any old dickhead that wanted one.
They've worked hard to clean up their act, and they have a very stringent membership scheme in place that generally prevents things like this from going on. But that scheme has obviously gone out of the window for this game and the resultant trouble was all too predictable unfortunately.
That game could easily have been played at Villa Park yesterday with more than enough tickets for both teams' regular fans, but the FA have a mortgage to pay so common sense has to leave the building.