While staying at the Eyrie, Sansa slaps Robin in his left cheek for destroying her snow replica of Winterfell. After the cut, Robin is holding his right cheek for the pain.
Bronn reminds Tyrion the deal they made long ago in The Pointy End (2011): "You once said if anyone ever asked me to sell you out, you'd double their price." Actually, what Tyrion said was "if the day ever comes when you're tempted to sell me out, remember this: whatever their price, I'll beat it." He did not offer to double the price.
After Melisandre gets out of her bath, she appears to have no water on her lower body at all, and the moisture on her shoulder disappears and reappears between shots.
Alliser Thorne claimed that the tunnel door is made from cold rolled steel. The process of cold rolling steel requires hydraulics, which hadn't been invented yet.
When Melisandre is bathing, she is not wearing her ubiquitous choker necklace. In season six, it is revealed she is actually centuries old and her true form is very old and decrepit, and her necklace provides a magical youthful glamour. Carice van Houten confirmed the error in an interview with Elle, stating "That was an oops moment."
When "The Mountain" is shown without a shirt or armor killing men at the beginning, he gets to the man whom kneels begging for mercy. The shot is shown from the front of the Mountain swinging straight downward. You can clearly see the blood spraying upward, originating from a single point to the right of the screen or to the "Mountain's" left, not even close to where his sword is making contact.