WebDAV servers where files are checked out, document editors with exclusive editing, and APIs that lock a record during a long-running job.
Why you would test it
Test the client's reaction to a resource someone else is editing: does it retry blindly, or tell the user who holds the lock and for how long?
What your client should do about a 423
Do not hammer it. Wait for the lock to be released — the body often says by whom or until when — or ask for the lock first if the API supports it.
423 versus the codes it gets confused with
423 vs 409 Conflict — 409 is a conflict with the resource's current state; 423 is specifically a lock held by someone else.
423 vs 412 Precondition Failed — 412 means your conditional request failed because the version changed — optimistic locking. 423 is pessimistic locking.
Endpoints that return 423
1 endpoint in this playground answers with 423. Every one is free, needs no signup, and can be called from the browser or with curl.