Have you ever tried opening an old Blender project from 2.79b in Blender 2.8 or newer and discovered your animations are broken because some of the bones in your armature are all messed up (wrong orientation)? This was driving me crazy yesterday. But it's easy to fix! Read the answer below to find out how.
I thank God for providing this simple solution! The offending bone was using a Copy Rotation constraint with settings that have changed in the new version of Blender.
Below is how the constraint looked on the messed up bone, using settings that were imported from 2.79b. The "Invert" options, which were needed in 2.79b for my particular setup, were no longer needed in Blender 2.8 or newer! Additionally, the "Mix" mode, which had defaulted to "Offset (Legacy)" to match the old "Offset" checkbox in 2.79b, was also causing problems. I found that setting it to "Add" produced the correct results, but you may require different settings for your particular setup.
Here is the corrected bone constraint (again, you may have to experiment with the settings for your particular setup, but this worked for me):
I hope this info helps someone as it did for me!