Continuity mistake: When Penny is making French toast and Sheldon walks into shot, the edges of the toast are not covered in egg whilst the rest of the toast is. About 30 seconds later, Penny asks Sheldon if he wants French toast and the edges are still not covered in egg. The shot cuts to Sheldon. In this shot we can see Penny's hands to the far edge of screen and fully raised showing us that she hasn't dipped the edges of the toast in egg yet. But in the next shot, we can see the whole of the toast is covered in egg, including the edges of the toast that were clear a moment earlier. (00:00:20 - 00:00:50)
Continuity mistake: When Howard is proposing to Bernadette she is holding a tray with two glasses on it. The glasses are next to each other, side by side for two shots of her. Then in the third shot the glasses move so one is behind the other. Then in the fourth shot they are side by side again.
Continuity mistake: Penny and Bernadette are working and Penny write her hair was in two braids with clips on top. But the second time we see her (same work shift), her hair is partially pulled back with bangs.
Continuity mistake: Penny asks Sheldon if the roommate agreement contains something about girlfriends, which Sheldon dismisses as unrealistic. However, it was revealed in season 2 that there is a cohabitation rider. This was revealed when Sheldon asserted that Leonard's girlfriend, Stephanie, was officially living with them.
Continuity mistake: When Sheldon sneaks into Leonard's bedroom while Leonard and Penny are sleeping, Penny has her arms out over the blanket and Leonard has his on his pillow in front of his face. When Sheldon leans over Penny only her hands are on top of the covers and Leonard's are tucked under his pillow.
Continuity mistake: When Raj and Leonard are sitting on the couch, Leonard is holding Watchmen and Lost in Space in his left hand and Young Sherlock Holmes in his right. In the very next shot when Leonard turns his head after Raj speaks, Watchmen has disappeared.