Undo / Redo

Not V for Vendetta but U for Undo!

You can undo module moves, additions, duplications and deletions, including movements involving linked modules or Fuzzy Swap.
Optimize Rack Space can also be undone, so experimenting with a different layout is a little less adventurous.

Find Undo and Redo at the top of the Edit menu, or press U to undo and Shift + U to redo while keyboard shortcuts are enabled. Multiple steps are supported.

The history is kept for your current tab and resets when you reload the page. It covers module placement changes, not every rack setting.