What did you do?

I read each frame of the gif, and then write them to a file.

What did you expect to happen?

That everything will work without a headache =)

What actually happened?

Some gif files wrote with artifacts and "optimize" doesn't work
pic2.gif

pic6.gif

I think compare successful and unsuccessful gifs will be right. Therefore I downloaded the gifs archive
"Bad" and "good" gifs tar archive

What versions of Pillow and Python are you using?

Pillow-4.1.1-cp35-cp35m-manylinux1_x86_64.whl
Python 3.5.2
Ubuntu 16.04.2 amd64

from PIL import Image, ImageChops, ImageSequence

f = Image.open('file.gif')
frames = []
framesIterator = ImageSequence.Iterator(f)
for frame in [ frame.copy() for frame in framesIterator ]: frames.append(frame)
frame.save('out.gif', save_all=True, optimize=True, append_images=frames)