I have spotted on the Wikimedia cluster a SVG image that cause RSVG to generate roughly 125MB of stderr output.
In includes/media/SVG.php we log that error using something like:
wfDebugLog( 'thumbnails', trim($stderr) );
trim() thus returns a copy of $stderr (aka roughly 125MB) which can't be allocated by PHP under its max memory constraint. That result in a fatal error and no log at all.
Which should probably substr() the $stderr before handling it. No point in logging the full error IMHO.