Windows 8.1 with Nvidia GT 720m
In my tests, the skinned mesh creates mirror errors. These errors recalculate differently when setting and reverting a setting in the modifiers list. As long as the skin modifier is there, the errors will keep applying to the mesh.
**Exact steps for others to reproduce the error**
1. Start with a vertex at zero on the x axis.
2. Add the mirrored modifier for the x axis, set to merge.
3. Add a skinned modifier with x set as a symmetry axis.
4. Extrude vertices, ensuring that center vertices remain perfectly on zero on the x axis.