If I try to embed a thumbnail of https://commons.wikimedia.org/wiki/File:Tulsi_Gabbard.jpg into a page, simply using "thumb" or "thumbnail" does not work.
So, for example, this usage does NOT work:
[[File:Tulsi Gabbard.jpg|thumbnail|left|This ought to be a thumbnail of the "Tulsi Gabbard.jpg" image, using the "thumbnail" argument.]]
However, this usage (adding a specific pixel width) DOES work:
[[File:Tulsi Gabbard.jpg|65px|thumbnail|Let's try 65 pixels.]]
And in contrast, thumbnailing with a different image ([[File:2LT Tulsi Gabbard Tamayo.jpg]]) works just fine whether I specify a width or not.
[[File:Tulsi Gabbard.jpg]] is 1.9MB and 3,190,000 pixels, which is smaller than [[File:Tomb of Safdarjung, New Delhi.jpg]] (which thumbnails just fine). So it's not a size issue. And I ran im.verify() from the Python Imaging Library against the image and didn't find any corruption issues, so the JPEG isn't corrupt. So I don't know why thumbnailing has broken for this specific file.
Not sure whether this is related to bug 41130.