No idea why, but strangely when looking at http://danbooru.donmai.us/posts/396760?tags=parent%3A280213 it resizes the image to 850px width, without showing the prompt to show the full version. I tried changing the image resizing setting, and this made the "Resized to 33% of original (view original)" appear, but after changing the setting back and reloading the page, the resized version is still being displayed.
I tried using both firefox, IE and chrome, same results.
I noticed because a few files I'd saved had the "sample-" prefix in the file name.
Here's a couple other posts that behave in the same way:
http://danbooru.donmai.us/posts/1551807
http://danbooru.donmai.us/posts/1552292
edit: I'm browsing about and it seems most images that are wider than 850px are doing this.
If this is already a known bug, feel free to close the topic, I'm not sure where to look that up.
Updated