When swapping node you can end up with two main nodes for an object, see result in screenshoot.

Steps to reproduce

1. have one article with 2-3 node assignment
2. create a new article with only one node assignment
3. swap node form #2 with one of the nodes in #1 that is NOT the main node.

Object #1 will now have multiple main nodes, but node #2 seems to be ok.