When Sheldon and Leonard are sitting at the cafeteria, Leonard's soda can keeps rotating between shots.
When Penny tells Leonard that Mike had moved on (before Leonard asks her out), the styling of Penny's hair changes depending on the camera angle.
When Sheldon pours soda into his glass, the fizz disappears too quickly between shots. He then pours in more, which makes lots of fizz that dissipates at a normal rate.
When we first see Penny eating the ice cream, some of it is visible near the top of the carton. It dips much lower without her eating any. Also, a bag of Teddy Grahams moves around the carton on the counter with no one touching it.
After Penny pours chocolate syrup into the ice cream carton and spills syrup over the side, subsequent shots have the carton turned so that the spill is no longer visible, then it is (further down the side of the carton), then it's turned away again, indicating different takes, but with the same carton.
During the lunchroom scene with Leonard and Sheldon, Sheldon shakes salt and pepper condiments on his food, however the shakers are grinders and do not shake the product on the food, but are supposed to be tipped upside down and twisting the tip to grind the minerals onto his food.