ERROR - Exception "null" (java.lang.NumberFormatException) while processing "<filepath>" (Referer:"null")

In fecru's log output we get error message triples like the following. We get about twenty per day. Over time the same files are repeatedly implicated, in this case header_background.png, but it's not just .png files and not all files of any one type in the repo. We see this for multiple repositories. Repositories are Subversion, accessed via HTTP on another virtual machine on the same VMware host.

2012-10-19 08:33:12,436 ERROR - Exception "null" (java.lang.NumberFormatException) while processing "/changelog-ajax/Reveal/trunk/reveal/public/images/header_background.png" (Referer:"null")
java.lang.NumberFormatException: null
at java.lang.Integer.parseInt(Integer.java:443)
[...]
2012-10-19 08:33:12,441 ERROR - /changelog-ajax/Reveal/trunk/reveal/public/images/header_background.png
java.lang.NumberFormatException: null
at java.lang.Integer.parseInt(Integer.java:443)
[...]
2012-10-19 08:33:12,441 ERROR - /changelog-ajax/Reveal/trunk/reveal/public/images/header_background.png
java.lang.NumberFormatException: null
at java.lang.Integer.parseInt(Integer.java:443)
[...]

More complete logs can be provided on request.