I assume this is a bug, but can’t be sure. Sometimes when I post a really thin image, the aspect ratio gets completely messed up and the image gets cut off. Take the following image. On preview, it seems like it will show up correctly. But then on the post itself it shows up zoomed in and cut off. You have to click the image to get it to show up properly.
More information: It seems like this might be happening delayed a bit. This image showed up correctly for a few moments, and then updated to the cut off view. Perhaps one of the post processing functions causes this to happen.