The exception log is refusing to give me a trace for this one, so I live-hacked wfGetAllCallers(false) into the exception message:

2012-10-24 18:46:03 mw70 commonswiki: [2f5ca212] /w/api.php?format=xml&action=upload Exception from line 112 of /usr/local/apache/common-local/php-1.21wmf2/includes/upload/UploadStash.php: key 'rsion=' is not in a proper format require/ApiMain::execute/ApiMain::executeActionWithErrorHandling/ApiMain::executeAction/ApiUpload::execute/ApiUpload::selectUploadModule/UploadFromChunks::continueChunks/UploadStash::getMetadata/UploadStash::getFile

jgerber wrote:

[USER-AGENT] => Wiki.java 0.26

so this must be http://code.google.com/p/wiki-java/source/browse/trunk/src/org/wikipedia/Wiki.java

i see no rsion in the code, on the other hand svn is at 0.27 and the released
version 0.26 did not have chunk support at all.
so someone must be using that file.

the page mentions

"""Bug reports can go to [[User talk:MER-C]] (fast) or the tracker
(slow). Before reporting character encoding problems, please display
the output in a JOptionPane to isolate your development
environment.""" http://en.wikipedia.org/wiki/User_talk:MER-C

possibly we can get them involved here.

after a quick look at the java code my guess is that the multipartPost function is broken, it does not use

Content-Disposition: file; name="chunk"; filename="foo.ogv"
Content-Type: application/ogg
data

for the chunk but

Content-Disposition: form-data; name="chunk"; filename="foo.ogv"
Content-Type: application/octet-stream
data

not sure if that causes parsing errors in php.

Is this still occurring?
If so, has a bug been raised in the clients issue tracker?