Mir: Why are there tears or glitches in my mesh?

Typically "tears" or "glitches" in a mesh are resolved by:

• Adjusting the Smooth Normals slider under the Fractal section of the effect
  This slider addresses flickering or tears found within the surface normals of the mesh.
  
• Updating your GPU's drivers
  On a Mac, GPU drivers are handled by OS X updates; if your GPU is adequate enough to work with Mir, consider updating your OS X to update its drivers. On a Windows setup, drivers are handled by whatever graphics card type you've got on your system (e.g. NVidia or AMD), in which case you'd need to go to their resources to check into how to update your drivers.
  
• Increasing the vertices, Supersample, and AO Radius
  In some cases, glitches or tears in the effect are natural byproducts of a texture warping itself to accommodate a heavily-distorted mesh. Increasing the definition of the this warping can help remedy these byproducts when visible.

If neither of those resolve the problem, the other factor tends to be that the graphics card you're working with isn't adequate enough to render out the mesh in Mir. With higher-resolution meshes (namely anything 4k or higher), a graphics card with at least 4GB of dedicated video memory is required.