Head mesh jumps into the eye mesh after joining armatures I joined the armature for eye animation with the armature for the head animation by Ctrl+J. Right after that the head becomes very small rotates 90 degrees and jumps into one of the eyes.
If I hide the head mesh, then the eyes become very big, move above head and overlap.
I managed to partially solve the problem by unparenting meshes from the armatures, then joining the armatures by ctrl+j and then parenting back the eyes. It all worked OK, but I cant parent back the head, as it get deformed in a very strange way.
I had a look at the file. This is caused by your shapekeys that uses vertex groups. Weighting is also assigning vertex groups. And that's what is conflicting here when you skin your headmesh. When i remove the vertex groups for the shapekeys before skinning then all works well.

So, first skin the head mesh, then create the shape keys for it.