callback_in_past
A requested callback time is at or before now, so there is no future instant to schedule. errors[] names callback_at.
Problem shape
Every non-2xx response is an RFC 9457 application/problem+json body. The type URI below is stable and machine-matchable, so branch on the final path segment (the code) and treat the status as redundant confirmation. See the error-handling guide.
{
"type": "https://docs.vocapable.com/errors/callback_in_past",
"title": "…",
"status": 409,
"detail": "…",
"instance": "/v1/…"
}