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.

CBlockIndex *pindexMostWork = NULL;
    CBlockIndex *pindexNewTip = NULL;
    do {
        boost::this_thread::interruption_point();
        if (ShutdownRequested())
            break;
        const CBlockIndex *pindexFork;
        bool fInitialDownload;
        int nNewHeight;
        {
            LOCK(cs_main);
            CBlockIndex *pindexOldTip = chainActive.Tip();
            if (pindexMostWork == NULL) {
                pindexMostWork = FindMostWorkChain();
            }
            
            if (pindexMostWork == NULL || pindexMostWork == chainActive.Tip())
                return true;
            bool fInvalidFound = false;
            if (!ActivateBestChainStep(state, chainparams, pindexMostWork, pblock && pblock->GetHash() == pindexMostWork->GetBlockHash() ? pblock : NULL, fInvalidFound))
                return false;
            if (fInvalidFound) {
                
                pindexMostWork = NULL;
            }