The atomic update feature described in SOLR-139 seems to depend on having an <updateLog /> configured in solrconfig.xml.
When used without an update log, the update commands like set or add don't result in an error and the transaction being aborted, but produce garbled documents instead. This is the case for both the XML and JSON formats for the update message.
I initially created some content like this:
Which resulted in this document:
Then I attempted to update that document with this statement:
Which resulted in this garbled document, with the fields that weren't updated missing: