Noticed this in my code review: https://petertodd.org/2016/segwit-consensus-critical-code-review#peer-to-peer-networking

Basically it looks like an attacker may be able to send nodes transactions with malleated witness data, which we don't consistently mark as possibly corrupted in AcceptToMemoryPool(). Result would be those txids being added to recentRejects, messing up propagation.

That said, I haven't actually tested this yet; about to go off to a conference, so if someone else wants to confirm for me that'd be much appreciated! Or if I'm wrong you're welcome to all laugh at my expense. :)

CC: @sdaftuar @sipa 

{
    return chainActive.Tip()->GetBlockTime() > GetAdjustedTime() - consensusParams.nPowTargetSpacing * 20;
}
bool PeerHasHeader(CNodeState *state, CBlockIndex *pindex)
{
    if (state->pindexBestKnownBlock && pindex == state->pindexBestKnownBlock->GetAncestor(pindex->nHeight))
        return true;
    if (state->pindexBestHeaderSent && pindex == state->pindexBestHeaderSent->GetAncestor(pindex->nHeight))
        return true;
    return false;
}
/** Find the last common ancestor two blocks have.
 *  Both pa and pb must be non-NULL. */
CBlockIndex* LastCommonAncestor(CBlockIndex* pa, CBlockIndex* pb) {
    if (pa->nHeight > pb->nHeight) {
        pa = pa->GetAncestor(pb->nHeight);
    } else if (pb->nHeight > pa->nHeight) {
        pb = pb->GetAncestor(pa->nHeight);
    }
    while (pa != pb && pa && pb) {
        pa = pa->pprev;
        pb = pb->pprev;
    }
    
    assert(pa == pb);
    return pa;
}
/** Update pindexLastCommonBlock and add not-in-flight missing successors to vBlocks, until it has
 *  at most count entries. */
void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vector<CBlockIndex*>& vBlocks, NodeId& nodeStaller) {