Why do I have a reputation change on my reputation page that says "serial upvoting/downvoting reversed"?

What does it mean when it says "serial upvoting reversed" or "serial downvoting reversed"?

When a single user continually votes (up or down) on many of your posts within a short period of time, this is referred to as voting fraud. This could happen for a variety of reasons, such as a user finding a user's great answer and visiting all of their posts to upvote them, or a user getting into an argument with another user and downvoting their posts indiscriminately in revenge. No matter the cause, this sort of systematic targetting voting is not considered normal behavior and the system will not allow it.

If it continues to happen between two users mutually or from one user towards another, or looks just plain suspicious in general, moderators and/or developers may investigate the matter. The votes involved will generally be invalidated as part of an automated process that runs every day and checks for signs of voting fraud. The votes may also be invalidated manually by the staff after an investigation. When the votes are invalidated, the reputation gain or loss from the votes is undone, which results in a record in the recipient's reputation history labelled "serial upvoting reversed" or "serial downvoting reversed".

What if I think I'm the victim of voting fraud?

If voting fraud is in the process of happening or just happened recently to your account, don't worry about it. You should wait at least 24 hours after noticing before becoming concerned, as the automated system should detect it and reverse it for you. If after this period you do not see any fix to your reputation, please contact the team using the "contact us" form located at the bottom of any page.

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 in most instances of that, they will have already been dealt with accordingly. You should in no way be concerned with reversal statements in your reputation history.