Revealing mistake: When Cliff Booth is driving Rick's Cadillac, in some angles the speedometer is showing 0 (zero) as the car is driven down the boulevard.
Revealing mistake: At one point when Cliff is driving the Coupe the ignition key which is located on the upper dashboard is in the off position.
Revealing mistake: As Cliff walks his dog down the street, and out of camera view, the hippies' car headlights can already be seen reflected in the back end panel of the parked car along the street (waiting for their cue). The car headlights are fixed, as the car stands still "parked" for several moments before pulling into frame when Cliff exits stage right.
Answer: To put some context into the scene, three members of the Manson family (Tex included) have broken into Rick Dalton's house and are holding Cliff Booth at gunpoint. Cliff, who is high on drugs asks "Are you real", to which Tex replies "I'm as real as a donut, mother fucker." Tex was trying to answer Cliff's question stating whether or not he was real, but in the most intimidating way possible, and I guess donut was the word that came to mind that described how real he was.
Casual Person