Other mistake: When Cliff fights Bruce Lee the doors are already dented before Bruce gets tossed into them.
Continuity mistake: Just before Rick's appearance on The FBI is shown, Rick is holding a cigarette in his left hand and a can in his right hand. It then cuts to Rick pointing at the TV, and the cigarette and can are both in his right hand.
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