When Matthew and Diana go horse riding, Matthew's horse changes between shots. During close ups/when Matthew Goode is clearly visible, it's a sturdier horse with tufts of hair around the hoofs. In wider shots/when the stuntman is riding, it's a sleeker horse, without the tufts of hair around the hooves.
In the tango dance scene of Matthew Clairmont and Diana Bishop, the tie is visible on and off the scene.
At the end of S01E04 Matthew storms out of the castle with warm vapour being breathed out with each exhale. His character is a dead vampire and wouldn't do this.