I've come across a problem with the field calculator. FYI, I'm using QGIS 22.214.171.124 via OSGEO4W on an XP box. Here's the issue:
When using field calculator to "Only Update Selected Features," all _unselected_ features (incorrectly) get a value filled. For example, if I create a new field, and set it equal to an existing field, the selected features get updated properly. However, all other features get the value of the first selected feature.