When Sonny, Soz and Herbie set off up the stairs with weapons to check for Richard's presence, they get to the top of the stairs in a different order than they started.
Before the drug scene when Richard is putting the drugs in the kettle you can clearly see under the gas mask Richard has long(ish) hair. While in every other scene he has very short hair.
When Al approaches the farm to lure Richard out, he is wearing black boots. After he is shot and we see his body lying on the ground, he is wearing brown cowboy boots.
Anthony is wearing Adidas Popper Pants in the film, including the flashback scenes. If the flashback scenes were from eight years years ago, it would have been 1996. Adidas Popper Pants weren't released until 1997.
Just before the famous confrontation scene between Sonny & Richard outside the garages - when the gangs car reverses upon seeing Richard, it parks up on the Speedbump on the left of the road. When Sonny steps out of the car, the car is now parked on the other speedbump on the other side of the road. When Sonny returns to the car after speaking to Richard, the car has switched back to being parked on the speedbump on the left.