What commands can not be reversed with the undo command?

Commands that typically cannot be reversed with the undo command include actions that permanently alter files or data, such as deleting files, formatting drives, or saving changes to a document without a backup. Additionally, operations that create new files or documents, like creating new layers or saving in a different format, often cannot be undone. Some software may also limit undo functionality after certain complex actions are performed. Always check the specific application's documentation for details on its undo capabilities.