When Polly and Midge are discussing the photographs, in one shot there is a second cigarette in the ashtray (besides the one Polly is smoking) which is not there in previous or subsequent shots.
When Perry is in Bongo's room listening to the tape, Bongo enters the room and takes a gun out of a drawer behind Perry's back while holding the door open. As Bongo is pointing the gun at the unsuspecting Perry, part of Paul Drake can be seen briefly in the hall through the open door. It is no surprise when Paul later bursts through the door and knocks Bongo down to rescue Perry.
Apparently, Paul was looking for Perry, so finding him would not be considered a Goof.
The younger sister Midge is 17, but the actress (Lorrie Richards) doesn't look like a teenager and is, in reality, older than the actress (Jo Morrow) playing her elder sister Polly by just over a week.
However, actors often play characters whose ages are quite different from their actual age; this does not constitute a Goof.
Midge, 17 years old, is smoking a cigarette while Polly and Perry are talking. At the time, the legal age for smoking in California was 18.
However, although the age to purchase cigarettes was 18, anyone can smoke them. Just as today when the age to purchase such products is 21, but there are still teenagers who smoke or vape.
When Perry finds the missing tape, a tape is already playing on the only tape recorder in the room. He takes the found tape and walks over to the recorder, and within a few seconds the found tape is playing on it. This is all done with the camera angle such that Perry's hands can't be seen. He could not have gotten that tape playing anywhere near that fast. He would have had to rewind the tape that was playing, remove it from the player, put the found tape on the player, and thread the tape onto the empty reel. This would have taken much more time than it took on screen.
In the opening scene, the guitarist's strumming hand is above the strings a good 2 to 3 inches - clearly not actually playing.
When Mason shows the two blackmail pictures to Jack Grabba, Grabba indicates that it is him in the second picture 'behind the doll with the dice' - except the person in the picture that has the dice is a man to the left with thinning hair. Midge Courtland is kneeling on the floor with her hands on her thighs.