Bree's surname is inconsistent (Daniel or Daniels) throughout the entire movie. The end credits read Daniel.
When Donald Sutherland writes notes on his legal pad about who the killer might be, it is not his hand but a stand-in who has veinless, pudgy, black-haired hands.
When Klute is driving Bree home, the truck following them changes to a regular car between shots.
The amount of tape in the take up reel of Klute's tape recorder when he stops it and rewinds while talking on the phone with Trask.
When Bree has drinks with one of her clients in his hotel, there is much clinking of ice on the soundtrack but no ice in the drinks.