The door to Kurt's apartment reads "3D" when, in the pilot, it read "406".
When Penny retrieves her mail, she opens the mailbox next to Leonard and Sheldon's (top row, second from left). However, in season one's The Cooper-Hofstadter Polarization (2008), Penny's mailbox was in the top right corner.
While Penny and Leonard discuss her expenses, the power cord attached to Penny's laptop changes position.
When Sheldon removes the lid to the snake can, he places it on the shelf. The lid then alternates between being on the shelf and in Sheldon's left hand for the rest of the scene.
Kurt's position in his doorway changes between shots.
Penny's electricity is reportedly off in her apartment but somehow her laptop getting and/or remaining charged.
Outlet in hallway common area.
Outlet in hallway common area.
The heroes play the Talisman board game, but they have too many character cards on a board (five, including one toad, while there are only three players).
Correction: Sometimes, when playing this and some other games, people play more than one character.
Correction: Sometimes, when playing this and some other games, people play more than one character.
When Penny and the guys are chatting while collecting their mail you can see that the zip on the back of Penny's skirt is undone.
When Sheldon is eliminating theaters from the whiteboard at the beginning of the show, he immediately strikes down 2 of them for not having "Icee Machines" - however, the markings on the board indicate that the theater showing the movie at 7:20 is "+IC", whereas the theater showing at 7:40 has "-IC". He eliminated the topmost theater on the wrong pretext.
When Penny is trying to detect the building manager outside Sheldon's door, they both listen for footsteps on the landing. Neither of them looks through the peephole above the dartboard.
Sheldon says that he has no immediate expenses coming up, until someone invents a way to fuse his skeleton with adamantium (referencing Marvel character Wolverine), saying that he hopes somebody is working on this. As a genius Sheldon would know perfectly well that adamantium is a purely fictional metal, making it impossible to do this in real life.
Howard refers to acid reflux as "burning a hole through my duodenum". However, the duodenum is the first segment of the small intestine, BELOW the stomach, and would have no part in acid reflux.
Sheldon lectures Penny on the impossibility of her paying him back sooner than she can, given a linear understanding of time and causality, indicating he finds the statement redundant, but earlier when he offers her the money, he himself says, "Pay me back when you can."