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.

list<CTransaction> removed;
            CValidationState stateDummy;
            if (tx.IsCoinBase() || !AcceptToMemoryPool(mempool, stateDummy, tx, false, NULL, true)) {
                mempool.removeRecursive(tx, removed);
            } else if (mempool.exists(tx.GetHash())) {
                vHashUpdate.push_back(tx.GetHash());
            }
        }
        
        
        
        
        
        mempool.UpdateTransactionsFromBlock(vHashUpdate);
    }
    
    UpdateTip(pindexDelete->pprev, chainparams);
    
    
    BOOST_FOREACH(const CTransaction &tx, block.vtx) {