There was a very nasty bug that caused inability to undo changes in some situations. Thanks to your feedback we have caught and tamed it! Now it is fixed so please upgrade RubyMine to 4.5.2.
Use ‘Check for updates’ from the main menu to install the update without re-downloading. You can also download the new version from RubyMine web site.
Develop with pleasure!
JetBrains RubyMine Team