Deliberate mistake: Whenever the Predator is in camouflage, it does not have its signature dreadlocks. This is confirmed in the DVD extras as being too difficult to include them when creating the visual effects.
Audio problem: In the final scene as the rescue helicopter is landing you can hear its engine spooling down; however the helicopter hasn't landed yet. As the helicopter takes off it does the opposite - you hear the engine spooling up, but the helicopter has already taken off.
Continuity mistake: When the Columbians are in the armoury near the beginning, they all look up as a skylight shatters. In the slow motion shots, you can see the leader turn away from the glass. But, then it shows him in a closer shot, and he's still facing the falling glass, and falling back away from it, shielding his face with his hands.
Character mistake: When Harrigan is watching the surveillance video by Bill Paxton he states that the helicopter is a silver Alouette when it is actually an AS-350.
Answer: The predators are honour bound hunters, and as such they will only attack a target if it is capable of defending itself (i.e, if it is armed, which is why it doesn't attack the boy in the cemetary). Although the police outside were armed as well, the Predator presumably attacked the drug dealers because they were a harder target (as the police were outside he could have just sniped them from the roof tops, but he had to get relatively close inside the building) and as such, killing them gave a higher honour. The obsession with Danny Glover's character is probably because he is quite a brave and heroic person, similar to a Predator (the way he saves the other cop by risking himself etc.), so the predator may have been studying him for a while. There's also more honour in killing someone like that (as he'd be a hard target) than there is in just killing a random armed guy.
Gary O'Reilly