Comment by Johnny555

One way it could be made insecure would be if it didn't use a good source of randomness, so the arrow direction could be predicted. A worst case example would be if the app just cycled through a fixed table of "random" choices like "L R L L R L R R", then the attacker could just watch the pattern and position himself in the line appropriately to make sure that his buddy Joe is the one to screen him. repositioning is easy, just dig through your bag and say to the guy behind you "I can't find my passport, you can go in front of me".