If you don't save changes (e.g. after adding a new entry), KeePass's auto-lock feature (e.g. after a certain idle time) pops up a "yes/no/cancel" question about saving changes.
If you're not around your computer to answer that question, then after a while KeePass stops responding. That is, the task bar icon is not clickable. Every right click option is grayed out. In the alt+tab menu, an unselectable white square replaces KeePass' icon.
Steps to reproduce: