I've experienced something quite strange. Gmail's cached version of an animated GIF of mine is super broken, it altered some frames dramatically. I already knew their GIF recompression was terrible, changing whites to off-whites and introducing noise, but this is really quite a big change:

Original:
http://bv-email-assets.s3.amazonaws.com/SOT-760/top-with-pigeon-sd-animated.gif

Google cached:
https://ci4.googleusercontent.com/proxy/8ecgZjaaiuKgDwiQhac920e0SQEKgn4tAWQBhzf21Ibtg5HVW9vS85OmLbhHkxxyN9B20q1fyBWGlzBKVFj1GsQJNv7wfHON84cPy_fYpNWdTCv20rBi5LBb60Q0umg8u94X49c=s0-d-e1-ft#http://bv-email-assets.s3.amazonaws.com/SOT-760/top-with-pigeon-sd-animated.gif