When mongorestore is run with object check (default in 2.4) it performs the check as follows:

 if ( _objcheck && ! o.valid() ) {

The problem here is that o.valid() is implemented as follows and consumes the Status object returned by validateBSON

bool BSONObj::valid() const {
    return validateBSON( objdata(), objsize() ).isOK();
}

mongorestore should instead call validateBSON directly and if Status.isOK() is false print Status.code() & Status.codeString() to help in pinpointing BSON corruption.