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.

return state.Error("out of disk space");
        
        FlushBlockFile();
        
        {
            std::vector<std::pair<int, const CBlockFileInfo*> > vFiles;
            vFiles.reserve(setDirtyFileInfo.size());
            for (set<int>::iterator it = setDirtyFileInfo.begin(); it != setDirtyFileInfo.end(); ) {
                vFiles.push_back(make_pair(*it, &vinfoBlockFile[*it]));
                setDirtyFileInfo.erase(it++);
            }
            std::vector<const CBlockIndex*> vBlocks;
            vBlocks.reserve(setDirtyBlockIndex.size());
            for (set<CBlockIndex*>::iterator it = setDirtyBlockIndex.begin(); it != setDirtyBlockIndex.end(); ) {
                vBlocks.push_back(*it);
                setDirtyBlockIndex.erase(it++);
            }
            if (!pblocktree->WriteBatchSync(vFiles, nLastBlockFile, vBlocks)) {
                return AbortNode(state, "Files to write to block index database");
            }
        }
        
        if (fFlushForPrune)
            UnlinkPrunedFiles(setFilesToPrune);
        nLastWrite = nNow;
    }
    
    if (fDoFullFlush) {