Revealing mistake: After the snake lady is dead, eyes open, Deckard rolls her head from camera right to camera left. Her eyes move. (00:59:15)
Revealing mistake: When Zhora dresses in Taffy Lewis' club she puts on high-heels. Later during the chase scene and when she flies through the windows, she is wearing normal boots. And when she is shot the second time, her right hand holds a control, probably for the blood bags on her back. (00:54:20 - 00:57:45)
Revealing mistake: Near the beginning of the film when Deckard and the other cop take off in the car to go to the police station, you can see the cables lifting the car up into the air. (00:10:05)
Revealing mistake: Near the end, just before Deckard goes into the final showdown apartment complex, a police car lands and asks Dekkard who he is. Deckard tells him and as the police car takes off, the strings holding it are painfully obvious. (01:27:40)
Revealing mistake: When Leon and Deckard are fighting in the street, Leon swings at Deckard and puts a hole through a steel vessel where steam blasts out. It's clear that the spot where Leon hits has been cut out and is visible before he hits it. (01:02:15)
Revealing mistake: As Zhora is crashing through the last window, after being shot by Deckard, there are dark pads visible around her lower arms. In few shots before, these are visible whilst crashing, but not whilst she is running or falling. (US theatrical version). (00:55:50)
Revealing mistake: When Deckard has the dream of a unicorn, it races towards the camera and you can see its horn. The horn is wobbly, revealing it's just stuck to a regular horse. [Note: this has been cleaned up for the Final Cut release, but can still be seen in earlier versions of the film.] (00:42:45)
Revealing mistake: When Dekkar retires the snake lady a spinner hovers overhead telling people to move on. At the back of the spinner you can see the cable that it's suspended from.
Revealing mistake: There are at least two rain related problems in the city exterior shots: it's noticeably misaligned when Rick and Gaff are outside Leon's apartment building (towards the right it falls straight down, yet towards the left it falls at an angle). The second is when Gaff encounters Rick and the now dead Baty on the hotel roof; despite previous shots showing no awning or any type of cover above said roof, Gaff is visibly dry and actually standing in front of the rainfall, only getting wet when he steps into the background.
Chosen answer: It's a more complicated question than you might think. Two versions of the scene were filmed, the main one, where Roy says "fucker" and an alternate, originally intended for use on television, where he says "father." Different versions of the movie use different takes. Of the three best known variants, the original theatrical release and the inaccurately-named Director's Cut both use the "fucker" line, whereas the Final Cut, the only one that Ridley Scott had full control over, uses the "father" line. What he's saying will depend on which version of the movie you were watching. Only you can answer that one.