Both readConcern level majority and snapshot require a storage engine that supports snapshot isolation. It is possible to bypass this check when:

1. Running a sharded cluster with standalone shards
2. Executing a readConcern level majority read operation with afterClusterTime

In this case, we skip calling replCoord->waitUntilOpTimeForRead() here and won't then call ReplicationCoordinatorImpl::_validateReadConcern() which will perform validation.

A proposal to fix would be to add the "if majority then isReadCommittedSupportedByStorageEngine()" check to waitForReadConcern() as a more universal check. We could then change the ReplicationCoordinatorImpl::_validateReadConcern() validation to be an invariant.