Of course, now it seems I can't reproduce it anymore... It was a global property condition like this :
global.property["dining_visited"] != true
This is pretty strange, last time the editor crashed right after I relaunched it and retried this manipulation. The only difference I see is the scripts I tried to copy this expression to have been "completed" manually and saved since, so maybe it wasn't directly related to copy and paste. I'll try to note every detail if I stumble upon this bug again.