Revealing mistake: When Danny is driving Mr. Zhao in his convertible at night, you can really see that the background is a bad fake and since even though Danny is driving with the top down, his hair and clothing does not get ruffled at all by the wind.
Revealing mistake: When Billy lies dead in the morgue and the President fixes his hair while saying, "That's better." we can see Billy's throat move as he swallows.
Star Crossed - S1-E6
Revealing mistake: About twenty minutes into the episode, Tru sits down next to Jen. Jen says, "I saw you in line in the cafeteria." Tru, with her hands at her sides, says, "Tru." Jen says, "Jen," and raises her hand to shake, at which point Tru reciprocates. But how did Jen know that Tru was introducing herself? After all, pretty much nobody else is named Tru, so in context it would have sounded like Tru was saying, "True," as in, "True; you did see me in the cafeteria."
Revealing mistake: When Mr. Hathaway contacts Scotty as they dig up his son's bicycle, a shot of Hathaway shows Los Angeles' US Bank Tower in the background. The series is set in Philadelphia.
Revealing mistake: In brother Justin's padded cell, the far wall is obviously a painted backdrop. It even flutters when the orderly and doctor walk in! The only solid part is a couple of feet at the end, that Justin leans up against.
Lightspeed - S5-E8
Revealing mistake: When the HIVE Five exit the museum and are intercepted by Kid Flash, Mammoth loses both his pants and the artifacts he has before KF whizzes past him. It's best seen in slow motion; at normal speed this mistake is hard to catch.