Other mistake: After Ethan crashes the Impala, he shoots at Keifer to make sure he's dead. Keifer's head and torso are on the hood of the car, but the shots break the back window on the driver's side. (01:13:05)
Other mistake: When the two main characters are fighting each other with a pair of scissors near the end of the film, it is obvious that they are using rubber scissors - what is supposed to be a solid metal object shakes side to side. (01:41:55 - 01:42:50)
Answer: As "Costa" was initially a suspect, the police took a DNA sample, which did not match any DNA on file - that was what they meant by "he's clean." It basically means that Asher was very careful at not leaving DNA evidence (other than that of others like Hart to frame them). Presumably the real Costa had no DNA profile linked to his identity on file either, or it could have returned a mismatch and revealed Asher's deception.
Sierra1 ★