in v0.12 https://github.com/bitcoin/bitcoin/pull/7081 (by @luke-jr) was merged adding `-bytespersigop` (with default of 20).

this is preventing simple bare multisig transactions from being accepted into mempool when the size is < 400 bytes.  
eg; https://gist.github.com/rubensayshi/ac4f617207f7a50559e85d61c05800be

from 1-of-7 and onwards they are accepted because the ratio of bytes:sigops is high enough.  
also having 2 or more inputs will work because it again tips the ratio of bytes:sigops over.

the reason mainly being that `GetLegacySigOpCount` calls `GetSigOpCount` with `fAccurate=false`.

It looks like changing `GetLegacySigOpCount` to use `fAccurate=true` would cause side effects in the few other places (`ConnectBlock`, `CheckBlock`, `CreateNewBlock`) and simply adding `fAccurate` as arg to `GetLegacySigOpCount` would affect the value stored in `CTxMemPoolEntry.sigOpCount` which in turn affects a bunch of other things.

{
        auto itPrev = mapOrphanTransactionsByPrev.find(txin.prevout);
        if (itPrev == mapOrphanTransactionsByPrev.end())
            continue;
        itPrev->second.erase(it);
        if (itPrev->second.empty())
            mapOrphanTransactionsByPrev.erase(itPrev);
    }
    mapOrphanTransactions.erase(it);
    return 1;
}
void EraseOrphansFor(NodeId peer)
{
    int nErased = 0;
    map<uint256, COrphanTx>::iterator iter = mapOrphanTransactions.begin();
    while (iter != mapOrphanTransactions.end())
    {
        map<uint256, COrphanTx>::iterator maybeErase = iter++; 
        if (maybeErase->second.fromPeer == peer)
        {
            nErased += EraseOrphanTx(maybeErase->second.tx.GetHash());
        }
    }
    if (nErased > 0) LogPrint("mempool", "Erased %d orphan tx from peer %d\n", nErased, peer);
}
unsigned int LimitOrphanTxSize(unsigned int nMaxOrphans) EXCLUSIVE_LOCKS_REQUIRED(cs_main)