Windows 7, NVIDIA GeForce GTX 1060 6GB
Broken: 2.77a, 2.78a
Short description of error
Cannot retrieve correct pose from pose library unless rotations modes of each bones are set as they were when the pose was added.
E.g., if I make a pose with all my bones' rotation modes set to quaternion and save it in my pose library, I would be able to retrieve the exact pose if I don't change the rotation mode of any of the bones. However, say if I changed the rotation mode of one of the bones to XYZ Euler, then I would not get the exact pose from the library.
Exact steps for others to reproduce the error
I attached a .blend file with an armature with some simple poses, I set rotation modes of all bones to quaternion when I saved them to the pose library, and you would be able to retrieve appropriate poses if you leave the rotation modes as they are (all set to quaternion).
If you change a rotation mode of any bone to anything other than quaternion and try to retrieve from the pose library, faulty poses would result.