Everything you do in HitFilm is logged, so that you can always undo any mistakes, even over multiple steps.
Undo & redo
You can undo and redo your actions using the undo/redo buttons at the top-left of the screen. This steps through your actions one at a time. You can also use the Ctrl+Z and Ctrl+Y shortcuts.
Hovering the mouse over the buttons will display the next action to be affected.
If you undo several steps and then perform a new action, you will not then be able to redo back to your project's earlier state.
A more powerful way to control your actions is to use the History panel.
The history panel shows a chronological list of all your actions, with your most recent at the bottom. You can undo and redo one step at a time using the buttons at the bottom of the panel.
Clicking on an action in the list will revert instantly back to that state. You can continue to click in the action list to jump to different points. This is a great way to instantly compare the state of the project before and after making a series of changes.
If you jump back several steps in the action list then perform a new action, and subsequent actions in the list will be permanently lost.
You can change the number of logged actions in Options.