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 (!ReceivedBlockTransactions(block, state, pindex, blockPos))
                return error("LoadBlockIndex(): genesis block not accepted");
            if (!ActivateBestChain(state, chainparams, &block))
                return error("LoadBlockIndex(): genesis block cannot be activated");
            
            return FlushStateToDisk(state, FLUSH_STATE_ALWAYS);
        } catch (const std::runtime_error& e) {
            return error("LoadBlockIndex(): failed to initialize block database: %s", e.what());
        }
    }
    return true;
}
bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskBlockPos *dbp)
{
    
    static std::multimap<uint256, CDiskBlockPos> mapBlocksUnknownParent;
    int64_t nStart = GetTimeMillis();
    int nLoaded = 0;
    try {
        
        CBufferedFile blkdat(fileIn, 2*MAX_BLOCK_SERIALIZED_SIZE, MAX_BLOCK_SERIALIZED_SIZE+8, SER_DISK, CLIENT_VERSION);
        uint64_t nRewind = blkdat.GetPos();
        while (!blkdat.eof()) {
            boost::this_thread::interruption_point();
            blkdat.SetPos(nRewind);
            nRewind++; 
            blkdat.SetLimit(); 
            unsigned int nSize = 0;
            try {