Deliberate mistake: After Ramsey shows Sansa her flayed northern friend, when it cuts to Davos riding the horse through the encampment one of the shots is flipped. Note his sword and his cloak's crossed straps have switched to opposite sides.
Deliberate mistake: Elsa's dress in Season Four is different from the dress she was wearing in the Season Three finale. The new dress is a different shade of blue, and has a completely different design, even though the present day events of Season Four take place mere moments after the end of Season Three.