wfGetSVGsize (in ImageFunctions.php) tries to detect the dimensions of a SVG file by looking for /<svg\s*([^>]*)\s*>/ in the first 4096 bytes of the file, acknowledging “Uber-crappy hack! Run through a real XML parser.” If it does not find the header by this method, it assumes the file is not a valid SVG file, and therefore, refuses to render its thumb on the image description page, replacing it with an icon.
This can break in many ways, especially with regards to XML comments: if the file starts with a big comment, the <svg> header might not fit into the first 4096 bytes of the file, or, if the comment contains <svg> it is mistaken for the real header.
Specifically, this happenned on http://commons.wikimedia.org/wiki/Image:Stub-icon_linguistics.svg (see version 16:57, 23. 5. 2008, marked 0×0) – the file begins with the full text of the GNU Free Documentation License, moving the SVG header outside the 4096 B limit.
Version: unspecified
Severity: minor