Continuity: 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.
Continuity: Bree's surname is inconsistent (Daniel or Daniels) throughout the entire movie. The end credits read Daniel.
Revealing mistakes: When Bree and Klute return to Bree's apartment from the market, the phone rings. Klute lifts the receiver halfway through the third ring whereupon the sound of the bell abruptly cuts off (rather than resonate a couple of seconds as it did during the first two rings), revealing the phone's ring as a studio recording.
Audio/visual unsynchronized: When Bree has drinks with one of her clients in his hotel, there is much clinking of ice on soundtrack but no ice in drinks.