Background validation is forced off until it can be finished. However the help text https://github.com/mongodb/mongo/blob/c31e309846609b4309a72868db7eafc189f839ca/src/mongo/db/commands/validate.cpp#L73-L80 and some invalid input cases can also mention it: https://github.com/mongodb/mongo/blob/c31e309846609b4309a72868db7eafc189f839ca/src/mongo/db/commands/validate.cpp#L150-L163