I received 4 downvotes within a short amount of time, I suspect it has been the same person. 

[![serial_downvoting][1]][1]

I think a mechanism should be installed that no user $u$ can downvote more than one post with a positive score of another user $v$. 

It's not dramatic, but it feels like "downvote trolling", and I think we should try to avoid this on mathoverflow.net


  [1]: https://i.sstatic.net/hm03Q.jpg