What is voting fraud?
Voting fraud is the systematic voting against correct voting rationales. The most common type of voting fraud is serial voting, which is when a single user continually upvotes or downvotes 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 certain specific users or from a single user, or looks just plain suspicious in general, moderators and/or Stack Exchange staff may investigate the matter and disciplinary action may be taken against the users involved in the fraud. Staff can also manually invalidate any votes they deem to be part of voting 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 may have seen a post of yours that was extraordinarily helpful to them and may have felt that going through your posts and upvoting them is an 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 sockpuppet account trying to game the system for extra reputation (which will 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, as with serial upvoting, visits their posts and downvotes them. Occasionally, other reasons for this occurring will prevail, like a prank 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. In most cases, the system should automatically detect and reverse it within an hour, possibly before you even notice it - there is no need to get concerned unless the votes have not been reversed in 24 hours.
If 24 hours have passed since the serial votes were cast and the suspicious votes have not been reversed, flag one of your affected posts as "in need of moderator intervention" and explain in detail what happened and when. If moderators see something concerning, they will escalate it to staff for review.
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 hour, a voting fraud detection script runs that looks for cases where a user has voted on (up or down) another user's posts many times. The threshold number is fairly low within a given amount of time (the exact details are not made public). When the system detects a pattern of votes that appear to be serial votes, those votes cast during that period 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 zero 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. This is because posts can be undeleted. While reputation earned is usually removed when a post is deleted, the votes remain, and if the post is undeleted, the reputation will also return.
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 "voting corrected" 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 "voting corrected" 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 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 a few 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 after you've reported voting fraud a bit frustrating, 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, "voting corrected" 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, please flag one of your own posts as "in need of moderator intervention" and explain the situation so they can investigate further.