How to manually mirror armature in blender 2.78a? In all previous version, we used to mirror armature by copying one side, and then center 3D cursor to zero, and duplicate, and scale by -1. However I notice that in version 2.78, (correct me if I'm wrong) scaling -1 on x seems to work differently than before as it will caused unwanted rotation on bones. Is it happening to everyone else, and what do I do to prevent it?
As Mzidare said, you can try Symmetrize. Here's a mirroring technique that circumvents this problem: 
You can enable X-Mirror, then extend from a central bone with E. The extension should occur symmetrically, and any changes to one should affect the other. I hope this helped :)