MongoDB currently disallows compound indexes whose components are single array elements selected by position from multiple arrays.

However, MongoDB does allow compound indexes whose components are single array elements selected by position from a single array. Furthermore, the resulting index is not multiKey.

Given that neither of these indexes should be multiKey, the check for multiple arrays at https://github.com/mongodb/mongo/blob/r2.4.8/src/mongo/db/indexkey.cpp#L232 may be a little too restrictive.