I have just seen this video of what has been called a runway incursion at LEBL (my hometown airport). A russian jet initiates a go-around after seeing an argentinian plane crossing runway 02:
It's not clear (as of 2014-07-07 1400L) whether ATC gave clearance to cross the runway. Anyway, my understanding is that even with ATC clearance you are required to check for incoming traffic and hold short of the runway if such traffic exists.
Seems like the taxiing crew either didn't look for incoming aircraft or they estimated they had enough separation to cross the runway.
So, is there any regulated separation distance (time/space) that can be used to tell who was responsible for this conflict? It's solely based on pilots' criteria?