wow, it sounds like you were actually able to provoke a real UCWE! Well, a real collision, at least.
So, did really none of the PUT commands result in an error? I would have expected at least one of them to emit a UCWE. Re-running the test and sending the output of each curl instance to a separate logfile would help answer this question. Also double-checking that curl emits errors to stdout when it gets a 500 or whatever HTTP error code UCWE maps to.
If you could, please do a file-check (with --verify) on the directory in question. With the dircap you show, the command would be "tahoe check --verify --raw $FW". I'm expecting to see a small number of shares of each version, for several different versions.
The file-check output will tell us, but what were the encoding parameters in use when you ran this test? I know from another ticket you were experimenting with parameters on the order of 40-of-50.. if the dirnodes (and other mutable files) were created with these same parameters, they'd be much more vulnerable to UCWE than with the normal 3-of-10 encoding. If that was a factor here, we might want to consider separate encoding-parameter configs for dirnodes (or perhaps for all mutable files), so that you can use safer 3-of-10 for them and more efficient 40-of-50 for immutable bulk data. (Note that protection from UCWE comes from small "k", whereas the usual reliability against server problems comes from having a large N-k).
(also, incidentally, when pasting large shell transcripts into a Trac page like this one, you should wrap the block with triple-curlies, so that Trac will not try to interpret the comments as WikiFormatting. And please attach other things as trac attachments instead of e.g. codepad links, because a few months from now when somebody comes back to look at this ticket, the pastebin will have expired and the contents lost)
