If you have made some edit you do not like and you have no clue how to undo it, you may want to return to the last saved version and try again.
The most obvious way to abandon changes and restart from a saved version is to click Exit button and open the subtitle editor again. However, an even faster way is to click Wrench icon above the editing panel and select Revert to last saved version item from the Tools menu:
When prompted for a confirmation, click Continue.
This will undo all the changes made since the last save and reload the saved version into the editor.