It happened to me that for a Scala source file a class name should have been imported while doing code completion. While the correct class name showed up in the completion dialog and the class name completion happened, no automatic import was done, which lead to a compilation error.

Internally the IDE should check if an import happened when one is requested (and not only imports but also other actions that change the structure of the source file) and if that did not happen it has to blew up - meaning at least an error has to be logged with all internal information available to give the user (and more important the IDE devs) a hint on what went wrong.