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.

{
    const uint256 hash = it->GetTx().GetHash();
    BOOST_FOREACH(const CTxIn& txin, it->GetTx().vin)
        mapNextTx.erase(txin.prevout);
    if (vTxHashes.size() > 1) {
        vTxHashes[it->vTxHashesIdx] = std::move(vTxHashes.back());
        vTxHashes[it->vTxHashesIdx].second->vTxHashesIdx = it->vTxHashesIdx;
        vTxHashes.pop_back();
        if (vTxHashes.size() * 2 < vTxHashes.capacity())
            vTxHashes.shrink_to_fit();
    } else
        vTxHashes.clear();
    totalTxSize -= it->GetTxSize();
    cachedInnerUsage -= it->DynamicMemoryUsage();
    cachedInnerUsage -= memusage::DynamicUsage(mapLinks[it].parents) + memusage::DynamicUsage(mapLinks[it].children);
    mapLinks.erase(it);
    mapTx.erase(it);
    nTransactionsUpdated++;
    minerPolicyEstimator->removeTx(hash);
}
void CTxMemPool::CalculateDescendants(txiter entryit, setEntries &setDescendants)