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);
    return nTransactionsUpdated;
}
void CTxMemPool::AddTransactionsUpdated(unsigned int n)
{
    LOCK(cs);
    nTransactionsUpdated += n;
}
bool CTxMemPool::addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry, setEntries &setAncestors, bool fCurrentEstimate)
{
    
    
    
    LOCK(cs);
    indexed_transaction_set::iterator newit = mapTx.insert(entry).first;
    mapLinks.insert(make_pair(newit, TxLinks()));
    
    
    
    std::map<uint256, std::pair<double, CAmount> >::const_iterator pos = mapDeltas.find(hash);
    if (pos != mapDeltas.end()) {
        const std::pair<double, CAmount> &deltas = pos->second;
        if (deltas.second) {
            mapTx.modify(newit, update_fee_delta(deltas.second));
        }
    }