As segwit is active on testnet, blocks will only be downloaded from NODE_WITNESS peers. Though, exactly those peers are disconnected after `BLOCK_STALLING_TIMEOUT = 2` seconds. I am running master (6e6ab2c)

This behavior is consistent across restarts. Sample [debug.log](http://pastebin.ubuntu.com/23058327/) and [getchaintips](http://pastebin.ubuntu.com/23058325/).

It is possible that I shot my own foot at some point, but it seemed smelly enough to create an issue.

{
    bool fClean = true;
    CCoinsModifier coins = view.ModifyCoins(out.hash);
    if (undo.nHeight != 0) {
        
        if (!coins->IsPruned())
            fClean = fClean && error("%s: undo data overwriting existing transaction", __func__);
        coins->Clear();
        coins->fCoinBase = undo.fCoinBase;
        coins->nHeight = undo.nHeight;
        coins->nVersion = undo.nVersion;
    } else {
        if (coins->IsPruned())
            fClean = fClean && error("%s: undo data adding output to missing transaction", __func__);
    }
    if (coins->IsAvailable(out.n))
        fClean = fClean && error("%s: undo data overwriting existing output", __func__);
    if (coins->vout.size() < out.n+1)
        coins->vout.resize(out.n+1);
    coins->vout[out.n] = undo.txout;
    return fClean;
}
bool DisconnectBlock(const CBlock& block, CValidationState& state, const CBlockIndex* pindex, CCoinsViewCache& view, bool* pfClean)