I had a typo in my POM: there was an extra trailing </project> tag in the XML. This made the POM not be valid XML.
Maven did not detect this error. While that may seem like a feature, it's really a bug, in particular because Maven will also happily deploy this invalid XML file.
It led to this interesting situation which had us scratching our heads for a while; see https://issues.sonatype.org/browse/OSSRH-22411 for details.
Recommendation: maven should throw an error if pom.xml is not valid XML.