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.

{
    bool fClean = true;
    CCoinsModifier coins = view.ModifyCoins(out.hash);
    if (undo.nHeight != 0) {
        
        if (!coins->IsPruned())
            fClean = fClean && error("%s: undo data overwriting existing transaction", __func__);
        coins->Clear();
        coins->fCoinBase = undo.fCoinBase;
        coins->nHeight = undo.nHeight;
        coins->nVersion = undo.nVersion;
    } else {
        if (coins->IsPruned())
            fClean = fClean && error("%s: undo data adding output to missing transaction", __func__);
    }
    if (coins->IsAvailable(out.n))
        fClean = fClean && error("%s: undo data overwriting existing output", __func__);
    if (coins->vout.size() < out.n+1)
        coins->vout.resize(out.n+1);
    coins->vout[out.n] = undo.txout;
    return fClean;
}
bool DisconnectBlock(const CBlock& block, CValidationState& state, const CBlockIndex* pindex, CCoinsViewCache& view, bool* pfClean)