Stephanie confesses to Leonard that when she X-rayed a patient, she saw one of her earrings that had fallen into him during surgery. In fact, it is a strict rule for all staff attending an operation in any U.S. hospital to remove all jewelry, piercings etc. before entering the operating theatre.
When Sheldon is talking to Wolowitz and Koothrapali at the cafeteria, stage lights are reflected in the water bottles on the table.
According to the mathematical formula that Sheldon gives for predicting the length of Leonard's relationships, his date with Penny should have lasted four and a half hours rather than three. Furthermore, the formula only works if Leonard's first relationship (with Joyce Kim) counts as number 0 rather than number 1.