When the patient Hannah stand up and turns around to show House her burn marks, House stabs her with an IV needle, and she turns around, acting surprised. However it's established that she can't feel anything, so she shouldn't have felt the needle prick and been surprised by it.
At minute 3:25 - 3:26, the blonde extra in the background is talking to another actress and then disappears behind Nurse Wendy. The extra then reappears after cutting to Dr. House.