Weird bone transformation

I tried to make a game and now i wanted to ad animations to the character. but when i tried to pose the bone in the left arm, the right arm gets transformed very weird.

Is this because i use a mirror modifier? hope you guys can help. :frowning:

Check the weighting of all the bones

No blend file supplied with your post so no one can check this for you, you’ll have to do this yourself

You could try to apply modifier. Always finish mesh before rigging it and apply all modifiers. Modifiers are uncompatible with armature, LOD and some other stuff in Blender Game Engine!