There should be a shared responsibility for the incident.
Fans shouldn't have gone on, that's a given... Some cock shouldn't have taken the corner flag...
But a big issue is the lack of stewarding for the final game to prevent such an occurance - the club know that people will run on, and from 85 minutes you knew that if a goal was scored, people would invade the pitch - yet there was a distinct lack of stewards to deal with an obvious problem.
Happy Days that we didn't get fined, but the club should learn from this and take on board for next seasons last home game and as others have said, if we go up, damn right i'll be running on - sod any fines.