The form refuses rules that would break the site:

• "Source path empty" / "Target URL empty" – both fields are required.
• "The homepage cannot be redirected" – / cannot be a source.
• "Source and target are the same URL — this would loop" – target resolves to the source.
• "Redirect #N already sends the target back to this source — this would loop" – a two-step loop with an existing rule.
• "A redirect for this source path already exists (#N)" – edit rule #N instead; each source can have one rule.
• "Source pattern is not a valid regular expression" – fix the pattern or switch the regex toggle off.

Even at request time a rule whose target resolves back to the same address is skipped, so a misconfigured rule falls back to the normal 404 page rather than looping.