Stupidity: Why on earth would the prisoners gang up on Hancock in the prison yard and threaten him with violence? They know who he is, and they know he is an invulnerable superhero and could take everyone of them down painfully with one hand. Even those who don't have personal experience of him would know from the media reports that he cannot be beaten by a human being. Don't tell me they think he is a reformed character or that he has somehow been 'weakened' by his imprisonment - they continue attacking him AFTER he has violently punished two, brutally humiliating two of them for merely threatening him - and they hurt themselves hitting him even when he just stood still and took it. Basic prison mentality - do not attack an opponent that you cannot beat. Losing a fight means losing status, losing face, and in a hellhole prison like the one in which Hancock is incarcerated, that can be - and often is - fatal.
Stupidity: During the gun battles with the "Tropic Thunder" actors, not one of the Flaming Dragon soldiers notice there is something strange about how the actors are firing their weapons (i.e, in the air, over the heads of the Flaming Dragons, etc.) or that there is no damage inflicted when the actors are shooting at or near objects in the area. Those two factors alone would tip off the Flaming Dragons that blanks were being used in the weapons of the actors.
Suggested correction: It wasn't so much as them believing they could beat him. But several factors here. The invincible superhero is suddenly in prison and not breaking out. It is reasonable to believe that a lot of inmates would think that perhaps he has been weakened and thus vulnerable and see this as opportunity for revenge. Even if that's not the case, basic prison mentality is to prove dominance and show no weakness. Attacking Hancock would demonstrate to the other prisoners that they did not have fear and were tough, even if they couldn't win as a way of proving themselves. Not only this, but the type of people in a prison tend to not be the most forward thinking type.
Quantom X ★
This correction is just plain wrong. Do you think Mike Tyson was ever attacked during his incarceration? Not a chance. I was in the Melbourne Remand Centre when a karate expert - he came third in the world championships - joined us after being banged up on his sixth dui charge. He was "ghosted" - that's an actual thing in prison - treated as if he wasn't even there. Not even the toughest of the tough would even acknowledge his presence.