The Keystone is described as a small orange gem, and appears as such when the Sea Devil arrives to steal it. When the Sea Devil later inserts it into his control mechanism, it is clear.
The height of the lead Sea Devil varies wildly, sometimes towering over the human characters by more than a foot, sometimes barely taller than them. This is especially evident during the action sequences.
When the Doctor and Yaz are sitting talking at the end of the episode, the side shots of the Doctor have her hair blowing in the wind, but from the rear her hair is perfectly still.
When The Doctor sees the flying Sea Devil boat, she declares such a sight to be "impossible." While The Doctor has encountered flying boats before, she does later refer to the Sea Devils not having ships, so her belief that such a sight is "impossible" seems to relate to the arrangement, not the actuality.
The positioning of the lead Sea Devil's translator necklace switches between shots, indicating that some shots may have been flipped.
The Doctor and her crew are able to swing from Madame Ching's ship the the Sea Devils', despite the Sea Devils' ship being at a higher elevation and there not being enough room for them to gain any momentum before attempting their swing.
The lead Sea Devil swings his sword at The Doctor, and the scene is played as if the sword came perilously close to her face, so much so that she had to turn and a sharp blue light is seen moving across her cheek. However, given the camera angle and distance, the sword should have been visible casting that defined of a light, and especially if its distance was so close that the reduced distance of turning her head slightly to the side saved her. It seems likely that the sword blade was meant to be comped in in post but never was, so The Doctor ends up dodging thin air with just a line of blue light on her cheek.
When The Doctor is arguing with the Sea Devil about the Keystone, she says the name of the character Ji-Hun, but her lips clearly don't match the spoken dialogue, and the volume and timber of her voice change for the name. The name was clearly overdubbed. Whether the name was changed after filming or the take was flubbed but patched and used anyway is unknown.
As Ying and Madame Ching face off after the discovery of Ying's Father, Madame Ching asks "You want to die out here, too?" When she asks "Like him?" her lips clearly don't move and it must have been dubbed in later.
The story takes place entirely in China (and the surrounding ocean) with a Chinese cast. It has been long established that the TARDIS crew are able to understand languages other than English due to the TARDIS's telepathic circuits acting as a translator, however, the telepathic circuits only remain in contact with a crew member while they're in the same time period. As soon as the TARDIS left 1807, Dan should have been unable to understand the Chinese characters.
The Sea Devils are shown speaking to one another using the translator necklaces that are used to communicate with land dwellers. The Sea Devils should all be able to communicate with one another naturally, without the use of a technological aid.
The Sea Devils are an aquatic species. They shouldn't need to have air-filled chambers under the sea.
Some of the Chinese characters speak with a Chinese accent while others speak with a British accent. If it is assumed they are all speaking a Chinese dialect but it is being translated through the TARDIS translation matrix for the benefit of Dan, Yaz, the Doctor, and the audience, then they should all have the same accent, either British or Chinese.