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.

{
            
            BlockMap::iterator mi = mapBlockIndex.find(hashStop);
            if (mi == mapBlockIndex.end())
                return true;
            pindex = (*mi).second;
        }
        else
        {
            
            pindex = FindForkInGlobalIndex(chainActive, locator);
            if (pindex)
                pindex = chainActive.Next(pindex);
        }
        
        vector<CBlock> vHeaders;
        int nLimit = MAX_HEADERS_RESULTS;
        LogPrint("net", "getheaders %d to %s from peer=%d\n", (pindex ? pindex->nHeight : -1), hashStop.ToString(), pfrom->id);
        for (; pindex; pindex = chainActive.Next(pindex))
        {
            vHeaders.push_back(pindex->GetBlockHeader());
            if (--nLimit <= 0 || pindex->GetBlockHash() == hashStop)
                break;
        }
        
        
        
        
        nodestate->pindexBestHeaderSent = pindex ? pindex : chainActive.Tip();
        pfrom->PushMessage(NetMsgType::HEADERS, vHeaders);