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Some GIFs aren't converting to the correct images to get the matching checksums.
This can be seen with running tg on dance.gif.

Some leads

For some image types, the presence of channels and bits values can be a bit confusing. As an example, GIF always uses 3 channels per pixel, but the number of bits per pixel cannot be calculated for an animated GIF with a global color table.
~ getimagesize manual

• ext/standard/image.c (PHP)
• gd_gif_in.c

Something related to colors or compression for animated GIFs seems to be going on.

This image causes the latest static build to segfault, regardless of OP/reply and SFW/NSFW:

http://s2.desu-usergeneratedcontent.xyz/mlp/image/1420/84/1420846635059.png

The image is 8-bit grayscale with alpha, a very uncommon format. I don't know for sure that that's relevant, but it seems like quite a coincidence if not.

PHP's GetImageSize() != ImageSX() and ImageSY() causing improper image dimension outputs.
This can be seen when running tg on dance.gif and comparing the dimensions of the output with the original thumbnail.