Upgrade to Zotero 4.0.25.3, now available, and also check your database integrity from the Advanced → Files and Folders pane of the Zotero preferences. Let us know if it happens again.
It indicates if there's something wrong with your database. It's relatively rare, but can occur (and then is usually easy to repair).
Not the case for you, though (thus "no errors").
Not the case for you, though (thus "no errors").