What is voting fraud?
Voting fraud is the systematic voting against correct voting rationales. Fraud most often happens with a single user continually upvoting or downvoting many of your posts within a short period of time. This is not considered normal behavior and the system will not allow it.
If mass voting continues to happen between two certain IP addresses (voting each other up) or from a single user, or looks just plain suspicious in general, moderators and/or developers may investigate the matter and disciplinary action may be taken against the users involved with the fraud.
When does serial upvoting occur?
Most often when you get unexpectedly serially upvoted, believe it or not, it's just a user trying to give you extra reputation. They saw a post of yours that was extraordinarily helpful to them and they feel that going through your posts systematically and upvoting them is the appropriate way of granting you additional reputation (the bounty system is the proper way to do so).
Occasionally, it also occurs between two users who have made an agreement to upvote each other, or between one user and a sock puppet account trying to game the system for extra reputation (which will often lead to suspension).
When does serial downvoting occur?
Pretty much any time serial downvoting occurs is when a user disagrees with something another user has posted, either as an answer or comment. The user then visits their profile and, like with serial upvoting, systematically visits their posts and downvotes them. Occasionally, other reasons for this occurring will prevail, like a joke being played on the user.
What if I think I'm the victim of voting fraud?
If the voting fraud is currently happening or has just happened recently, don't worry about it. You should wait at least 24 hours after the voting has occurred before becoming concerned. In most cases, the system should automatically detect it and reverse it for you within 24 hours. You don't need to take any action during those 24 hours, as all that moderators can do during that time is tell you to wait for the automatic detection script to run (they can't run it for you just this once).
If the 24 hours have already passed and the suspicious votes have not been reversed, you can then flag one of your own posts for "in need of moderator intervention" and explain in detail what happened.
It is strongly preferred that you do not ask about serial voting on a site's meta. The details of the investigation are private and will not be divulged, and there's nothing that normal users can do. Such meta posts often lead to inappropriate speculation as to the cause, who the voters are, etc., and they also tend to result in people engaging in additional targeted voting.
How does the system detect voting fraud?
Every night around 03:00 UTC, a voting fraud detection script is run that looks for patterns such as these. It basically looks for users who have voted on another user's posts many times. The threshold number is fairly low within a given amount of time (the exact mechanics are kept secret). When the system detects this pattern, all of the votes cast from that user to the user affected are reversed, and a "voting corrected" event is added to the affected user's reputation history to indicate what has occurred.
The value of the reversal could be anything, as it is the combined amount of all the votes being reversed (up and down, although most serial voting occurs in one direction). If you had reached the reputation cap, the value may show up as blank if none of the votes being reversed actually affected your reputation on that day.
Does the system detect voting fraud on deleted posts?
Yes. If you serial vote on posts and one or more of them end up deleted at a later time, the script will still reverse those votes if they are detected.
A regular user may notice this "feature" when one runs across a spam post, checks the spammer's account, then discovers and downvotes a series of "answers" having the same canned spam content. Even if all the posts are flagged and further removed, some time later one can find a series of +1 undownvoted entries in their reputation history marked by the time when the vote reversal script typically runs (example).
Why don't I get to keep the reputation?
The reputation was removed because serial voting is not proper behavior and it is not allowed. The votes were completely invalidated by the system and thus the reputation gain from them was also invalidated. The only way you can gain this reputation back is to go post some more and get some legitimate upvotes on those posts.
Should I be concerned about reversal statements on my profile?
No, not at all. It's only an indication of reputation change. After all, we can't control the actions of other users. It's very rare where we'd run across a user who was committing the voting fraud themselves on their own account, and if we believe you're doing that, you will have already been warned separately or suspended. In no way should you be concerned with reversal statements in yours or anyone else's reputation history.
I've flagged my post for moderator attention to report voting fraud; how does the investigation process work?
Moderators have access to some information that can identify patterns of voting fraud (but doesn't disclose full voting information). When a moderator receives a flag reporting voting fraud, they will investigate it using their tools.
If the moderator who processes your flag agrees that there is, or may be, voting fraud, then they will escalate the issue to Stack Exchange staff, who are the only ones with access to full records and the ability to manually invalidate votes. It may take a long time for the issue to be fully resolved by staff due to the backlog of requests. While staff generally try to keep the backlog down to days or a couple weeks, there have been times when the backlog has been more than six months.
You will not be provided any additional direct response with regards to your flag or the voting fraud issue, other than the moderator dismissing your flag as "helpful" or "declined", which they usually do when they make the choice of whether or not to escalate to staff. Such flags are generally marked "helpful" if the moderator feels you raising the flag was reasonable in that instance, but it does not necessarily mean that they agree that there is a voting fraud issue. The moderator may, or may not, provide a brief text response in addition to dismissing your flag.
The fact that you don't get any additional direct information can make the process a bit frustrating after you've reported an issue, as you won't get any additional status updates or a statement that the overall process is complete. The only indirect information you may get is, if the investigation results in vote invalidations, reversal statements in your reputation log. In some instances, you may also see users suspended for "voting irregularities", but suspensions are not handed out in every instance. No information is provided as to the resolution of the investigation other than those possible effects.
What else should I know about this subject?
Reversals trigger a reputation recalculation.
Whenever serial voting gets reversed, all days involved in the serial voting will be recalculated as if the serial votes had never happened, and any reputation you might have lost due to those serial votes hitting the reputation cap will be given to you at that time. Serial voters will not prevent you from gaining reputation that you deserve. This also means that any other inconsistencies in your reputation history will be fixed as well.
(Under some rare cases, your reputation may not be recalculated immediately, resulting in an incorrect reputation total. However, the next time your reputation is recalculated - generally, through one of your posts being deleted or undeleted - any reputation you lost due to the serial votes hitting the reputation cap will be given back.)
The votes can be re-cast.
Votes reversed by the detection script can be re-cast by the user at a later time, so long as the user does not again engage in serial voting which causes them to be reversed (yet again).
There is no automated punishment or notifications.
Currently, the serial voting script only reverses the votes, and takes no other action except putting an event in your reputation history. There is no automated ban for users who engage in serial voting, and the only form of notification they receive that their votes were reversed is "+1 undownvoted" events in their reputation history if they had downvoted answers (if they only downvoted questions or had only upvoted things, they will not be notified in any way).
Continual abuse of the system which causes multiple reversals can lead to a suspension, but the review process for such a suspension is completely manual. No automated flags are ever generated for this behavior, though moderators do have access to some statistics that can help fight repeat offenders. If you find you are the victim of serial voting multiple times within a short time span, feel free to flag one of your own posts for moderator attention and explain the situation so they can investigate further.