Continuity mistake: When Sheldon is in Penny's apartment trying to get her to tell him why her relationship with Leonard failed, the yellow tee-shirt on the red pillow next to him keeps changing position.
Continuity mistake: When Sheldon is questioning Raj and Howard in the cafeteria, the bottle of water is on Raj's tray faces the camera with the barcoded side. But in one closeup it rotates to show the logo-branded side.
Continuity mistake: When climbing the stairs at the start, Penny and Sheldon turn the corner and carry on the conversation. Camera cuts and now they turn the corner again and continue their conversation without breaking it.