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 (!boost::filesystem::exists(GetBlockPosFilename(pos, "blk")))
                break; 
            FILE *file = OpenBlockFile(pos, true);
            if (!file)
                break; 
            LogPrintf("Reindexing block file blk%05u.dat...\n", (unsigned int)nFile);
            LoadExternalBlockFile(chainparams, file, &pos);
            nFile++;
        }
        pblocktree->WriteReindexing(false);
        fReindex = false;
        LogPrintf("Reindexing finished\n");
        
        InitBlockIndex(chainparams);
    }
    
    boost::filesystem::path pathBootstrap = GetDataDir() / "bootstrap.dat";
    if (boost::filesystem::exists(pathBootstrap)) {
        FILE *file = fopen(pathBootstrap.string().c_str(), "rb");
        if (file) {
            boost::filesystem::path pathBootstrapOld = GetDataDir() / "bootstrap.dat.old";
            LogPrintf("Importing bootstrap.dat...\n");
            LoadExternalBlockFile(chainparams, file);
            RenameOver(pathBootstrap, pathBootstrapOld);
        } else {
            LogPrintf("Warning: Could not open bootstrap file %s\n", pathBootstrap.string());
        }
    }
    
    BOOST_FOREACH(const boost::filesystem::path& path, vImportFiles) {