This help page on Serial voting reversed states:
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.
This very suspicious string of upvotes was reversed a few minutes ago.
However, a similar voting pattern occured the day before, which has not yet been reversed:
And a few days earlier (also not yet reversed):
Besides, the vote pattern on this particular answer has raised some eyebrows...
According to the same help page:
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".
Did the "automated process that runs every day and checks for signs of voting fraud" fail? Did the voting fraud escape the mods' attention?