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.
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.]
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.