The Bells of Saint John - S7-E8
Continuity mistake: When the Doctor and Clara are on the rooftop terrace, there's a distinctive design on the Doctor's bow tie, but when he's in the coffee shop the bow tie's design differs, and when he runs outside to ask if Clara's okay it changes back, then it changes again when he's in back the shop. Additionally, when the walking base station Doctor uploads Clara it's wearing a bow tie, and its design also changes on the motorbike and in Kizlet's office.