I post a majority of my answers to GameDev. Due to the context of my answers, I often create small diagrams or animations to help other users. Today, I noticed that an animated GIF I uploaded was not animating. Further tests confirm that this appears to be a bug.
I have also confirmed the same behaviour on Stack Overflow, English Language Learners, Science Fiction & Fantasy and here on meta.
Are you sure it is not just your computer? Yes. Animations work fine, otherwise. Furthermore, answers where I have previously uploaded animated diagrams are still animating.
Why can't you just provide anonymous links? Actually, I can. It appears that if I upload an image via an anonymous account, and then use the direct link to upload the same image to the Stack Overflow account, it works. Regardless, this process is a bit obtuse, and it still highlights the presence of a bug.
Could it be an issue with the imposed size limits? While animation naturally infers an increased size limit, these animations all fall under the maximum of 2MB. I have confirmed this problem on animations ranging from approximately 50kB to 500kB.
Changing the size of your uploaded image with '-s', '-m' or '-l' cancels animation? Sounds like you have your answer. Not by a long shot. I made the mistake of assigning the "medium" variant of my original images, for the purpose of demonstrating the difference in this question. This has been corrected, but it does not solve the actual bug I am observing. Both of the below examples come from the exact same image, and should both be animating.
Are you sure it is not a problem with imgur? Yes. If I upload the same image via an anonymous account, it animates. I directly uploaded the left version of the image, where I uploaded the right version directly to an anonymous imgur account and provided the link.