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.

{
    LOCK(cs);
    _clear();
}
void CTxMemPool::check(const CCoinsViewCache *pcoins) const
{
    if (nCheckFrequency == 0)
        return;
    if (insecure_rand() >= nCheckFrequency)
        return;
    LogPrint("mempool", "Checking mempool with %u transactions and %u inputs\n", (unsigned int)mapTx.size(), (unsigned int)mapNextTx.size());
    uint64_t checkTotal = 0;
    uint64_t innerUsage = 0;
    CCoinsViewCache mempoolDuplicate(const_cast<CCoinsViewCache*>(pcoins));
    LOCK(cs);
    list<const CTxMemPoolEntry*> waitingOnDependants;
    for (indexed_transaction_set::const_iterator it = mapTx.begin(); it != mapTx.end(); it++) {
        unsigned int i = 0;
        checkTotal += it->GetTxSize();
        innerUsage += it->DynamicMemoryUsage();
        const CTransaction& tx = it->GetTx();
        txlinksMap::const_iterator linksiter = mapLinks.find(it);
        assert(linksiter != mapLinks.end());
        const TxLinks &links = linksiter->second;
        innerUsage += memusage::DynamicUsage(links.parents) + memusage::DynamicUsage(links.children);
        bool fDependsWait = false;
        setEntries setParentCheck;
        int64_t parentSizes = 0;
        int64_t parentSigOpCost = 0;
        BOOST_FOREACH(const CTxIn &txin, tx.vin) {