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.

if (outsBlock.nVersion < 0)
            outs->nVersion = outsBlock.nVersion;
        if (*outs != outsBlock)
            fClean = fClean && error("DisconnectBlock(): added transaction mismatch? database corrupted");
        
        outs->Clear();
        }
        
        if (i > 0) { 
            const CTxUndo &txundo = blockUndo.vtxundo[i-1];
            if (txundo.vprevout.size() != tx.vin.size())
                return error("DisconnectBlock(): transaction and undo data inconsistent");
            for (unsigned int j = tx.vin.size(); j-- > 0;) {
                const COutPoint &out = tx.vin[j].prevout;
                const CTxInUndo &undo = txundo.vprevout[j];
                if (!ApplyTxInUndo(undo, view, out))
                    fClean = false;
            }
        }
    }
    
    view.SetBestBlock(pindex->pprev->GetBlockHash());
    if (pfClean) {
        *pfClean = fClean;
        return true;
    }
    return fClean;
}
void static FlushBlockFile(bool fFinalize = false)