Just now I had a slew of upvotes directed at me. This upped my reputation by 200. I know that this is something called "Santa Claus" in action. From what I've heard, some users will sprinkle upvotes to all the good users. Should we allow this to happen? Check my actual Arqade profile, not my meta one.
If a single person upvotes or downvotes another single person too frequently in too short a time-span, it will trigger a "serial vote reversal". All those votes will be invalidated and it'll be like they never happened.
The system is automatic, so I really wouldn't worry about it too much. It'll handle what it's meant to handle and everything else probably isn't a big deal.
If it's serial voting (voting on many of a single user's posts because of the user instead of the content quality or correctness), it will be reversed by the system. The bottom line is, you don't need to worry about it.