Here's what I did:
The above led to a new collection named thisshouldfail, with the config-set as gettingstarted. This call should have failed as there was no configset by that name. Instead, it picked up the only config set it found and used it.
There's more to this. I'm not sure how related this is but looks like it to me.
This led to a 900M (and growing) log file in addition to 100% CPU until I killed Solr.