Mirrored UV map is breaking up textures per-face Uv map was mirrored, but then every single face in an island is ripped seperately. 
The map is in the perfect place but the textures per face are acting wonky.

Screencast of the situtation
(I'm sorry i wasn't able to upload a gif) 

For more details:
The mesh is downloaded from blendswap, comes Unwrapped and Textured.

I modified one side of the mesh using edit and sculpt modes. X-mirror was on, but sadly somewhere i deleted some faces and i think it stopped copying my edits over to the left side. 

The error is on the left side.
I deleted the left side.

I selected the right side and did shift+d scale x=-1
but ofc that only uses same UV's as right side while the textures aren't exactly the same. 

I couldn't re-unwrap using U>Unwrap because the mesh is changed, and the resulting unwrap is different.

so I selected right side, separated by selection, mirrored it using mirror modifier, deleted the original side (lol must be tiring to read this, sorry.) and selected this new object in object mode, shift+selected the original object, did ctrl+L Transfer UV maps.
That last step must have made the errors.

I manually edited the new left side UV's to fit the texture map perfectly. BUT, the textures seemed off for this one side. Every face shows gradients. almost like if the normals weren't set to smooth. But they are.
The maps are in the perfect place, but when i select islands in the UV editor, it doesn't select the whole island, just random chunks of faces.
And the same happens when i select vertices.

I tried W>Remove UV doubles, at a low threshold too. It doesn't work.

Any way to salvage this? Is this a bug?

In the end I am deleting half the whole mesh and just going with mirrored textures... It doesn't really matter that much, I'm just surprised at the mess haha.
It's becouse faces in that place are "rotated". Let's i describe it in image:



your vertex number 1 is on position of vertex 3, n.2 on position of 1, n.3 on pos. 2.

so it look like all vertex on right place, but this places are wrong

Look at my another example. As you see all faces are in right place, but selected face is rotated at 90 degrees. Same thing in your model, but many of faces are rotated.



So if you have original model - try to transfer it UV map using Data Transfer modifier