I encounter this problem a few times when I was making an animation. In the first senerio I was to make the first and the last frame of the animation exactly the same so the animation loops smoothly. I did this by copying all the keys recorded in the first frame and paste them in the last frame. But I found that after pasting, the two frames were not exactly the same, in my case, the foot and leg differed in a very tiny angle and position, but that were still noticable and the numeric values for rotaion and translation were truly different. I try to paste the leg part again but there is no use. I then had to specifically enter the numeric value to solve the problem.
In the second senerio, I wanted to add a frame from another animation at the end of this animation, the images are different so there were also slot image keys. After copying, I noticed the slot image keys were not copied at all and I had to change all of then manually in the tree, also the leg part is not exactly the same (I should say not even close, unlike the first senerio), I then try to re-copy all they key from the leg only and fortunately this time it works, so eventually problem solved.
Is this a bug? Please exam it.