Do you know what this is?

This is a problem of different vertex order in blendshapes. This can be caused for many reasons. For e.g. the topology is same but one of the meshes was edited)

I remember the times when I needed to add extra loops to head with the already created blendshapes. And to not break the vertex order I needed to do all operations in exactly the same sequence for each blendshape. It was a nightmare :)

How can we fix it?

With this add-on you can simply copy and paste verts, edges, faces ids orders between meshes with the same topologies in two clicks in pie menu. Demonstration

Optimized and works really fast!

Сopying and pasting takes about 20 seconds on a mesh with 3 million triangles. On a low poly model, it will take a split second.

Blendshape usage

Blendshapes is very powerful tool not only for animation but also for modeling and sculpting (morph target technique). Imagine you have a setup model in Blender with names, materials, textures, applied but you want to tweak it in some other software. With blendshapes you haven’t to setup from scratch an imported model with changes. You can just use it as  blendshape applied to setup model. And if it turns out that something wrong with vertex order, you can easily fix it using this add-on.