In a scene at the Cheese Cake Factory where Sheldon is explaining why the single-decker burger is better than the double-decker burger from Big Boy, you can see him taking a bite and the burger is almost all gone and then it switches to a different angle as Leonard is speaking and suddenly his burger is much larger again.
When Leonard and Leslie get up and rush to the bedroom, he leaves his cello on the floor, and the two folding chairs and their music stands are left where they were. When Sheldon comes home, these are all missing.
Sheldon and Leonard are going down the stairs discussing Penny's reaction to Leonard's night with Leslie. On each flight of stairs, Sheldon is in the lead when they go out of view behind the elevator shaft, but they have switched positions when they come into view again.
After Leslie puts the shattered pieces of the frozen banana into her bowl, she pours milk on it. The banana is fresh from being flash frozen by liquid nitrogen, it would have frozen the milk.
When Sheldon wakes up in the morning after Leslie stays the night, his head is on the end of the couch nearest to the door. This is in direct contradiction to what he tells Penny in the future about how the headboard is oriented away from the door to serve the ancient imperative of protecting oneself against marauders. However, given that in the previous scene he is sat not knowing what to do, it can be assumed that he sat there until eventually falling asleep out of exhaustion, without having planned it out, as evidenced by him still being sat in 'his spot'. Alternatively he may have learned of the ancient custom between this and the future episode.
Leslie tells Leonard to wear protective goggles. Regular prescription eye glasses are not enough to provide protection. Protective goggles will cover regular glasses, which is the reason they are so large and have grooves where eyeglass handles pass through.
The morning after Leonard and Leslie have sex, Leonard picks up his glasses, putting his finger through the place where the lens should be.
Neither Leslie nor Leonard's bows are making contact with the strings while playing their instruments. In fact, Leonard's bow is being held upside down.
When Leonard and Leslie are alone in the apartment practicing the music for their string quartet Leonard's cello bow is facing the wrong way. Meaning his cello should not be making any noise.
After Leslie smashes the flash-frozen banana, the bigger pieces fall out. But when she says her lines about not having a knife, not only are there big pieces on the glass container, but there are also 2 banana peelings.
When Leslie says to Leonard, "if you stick electrodes in a rat's brain, give him an orgasm button, he'll push that thing until he starves to death."
She leaves out the part about giving the rat a choice between a button that will feed him and an orgasm button. Without this crucial detail, her analogy doesn't make sense.
When Leslie and Leonard stay behind to practice the middle section of the string quartet, the music sounds exactly the same as when all four musicians were playing.
Both Leonard and Leslie's fingering of their instruments is clearly off compared to the music.
Leslie wears protective goggles when working with the liquid nitrogen, but fails to wear protective gloves even though her hands come into close proximity to the liquid nitrogen and the metal tongs she uses are not properly insulated.
Leslie tells Leonard that when he looked at Penny his pupils dilated and unless he was a heroin addict that is a sign of sexual attraction. Heroin addicts do not have dilated pupils. Opiate users, including heroin addicts, have pinpoint pupils. Even though Leslie is not a Pharmacist, Pharmacologist or Physician, she should know this.