Operating system:Windows 10
Broken: 2.83, also tested in current 2.9 alpha with same behaviour
Worked: Fairly new to blender, so I've been doing alot of things that it might not expect.
**Short description of error**
Mirror modifier causes a number of issues when using an armature. As an example, with a normal mesh(assuming proper topology for efficiency with this report), you would expect the transforms of one side of the mesh to be very properly mirrored, but they are often very wacky and out of place, often resulting in the mirrored side becoming a mess of spaghetti limbs as soon as you begin to pose. The simplest fix is to apply the mirror modifier.
Another good example is when you have a character which you need to add clothing to. If that clothing has a mirror modifier, it will typically base all of its movement on the non-mirrored side and not deform the mirrored side whatsoever.
If you have IK restraints, however, I have noticed that the clothing will not deform at all, instead simply rotating based on the movement of the bone.
I have attached a file. It is my first project, and as such it includes all of the various screwups and several meshes that were discarded. The layers you will need to enable are the Retopo, Armature, Pantsu, Suit, as well as Eye and Eye.001. the file is about 200 megabytes. The addons that I have are Retopoflow, BY-GEN, F2, and Rigify. Although, I do not believe you will need any of them. You will be missing images that I was using for reference, as well as the iris texture. I will include the iris texture for those of you who wish to have it.
Steps for reproduction with this file
1. Parent "Suit" or "Pantsu" to the armature(with any weighting option)
2. try to pose and notice the issue.
For other files:
1. Make an object with a mirror mod
2. Add Armature
3. Parent object to armature(with any weighting option)
5. Notice same and similar issues.
If you guys want to put a band-aid over it, I would suggest adding a warning when you attempt to parent an object to an armature and they have a mirror modifier, or prevent them from parenting at all until the mirror is applied.
If I for some reason didn't increment after finding this out, The pants should still have a mirror mod on them.