Rollback: `rollback n` (0–49), `show | compare rollback n`, rescue configuration

Every time you run commit on a Junos device, the software keeps a copy of the configuration that was active before. These saved copies form a numbered history. The currently active configuration is rollback 0, the one committed just before it is rollback 1, and so on back to rollback 49, so Junos keeps up to 50 versions in total. This history is one of the biggest practical advantages of Junos: undoing a bad change is a single command instead of a frantic retyping session.