Recently, a user posted several questions within the space of about a half hour. I downvoted four of them because I felt that they were poorly researched, and because they were out of the scope of the FAQ. The votes were close together because of the timing of the questions. Will the revenge down-vote script kick in? I feel that all of my votes were valid.
The downvote script's functionality is deliberately kept a secret. After all, if details about when it triggers were available, people would basically dance around that line, never tripping it. As such, we cannot reveal the exact criteria of whether or not the downvote script will trigger for any instance.
As long as you're doing normal site activity, and not, say, actively seeking out posts across the site to downvote, then you shouldn't run afoul of the voter fraud script. It's not all that different from the cases where someone will have posted a lot of good answers in a short time - it may simply be a natural incident for one user to upvote all of those answers. As long as you're acting naturally, it shouldn't come up for you.
@Simchona, I totally agree. Those questions showed lack of research and effort. To downvote, was the correct thing to do. Perhaps, if the down-vote script does kick in, it would show the one flaw in machines -- they can't tell appropriateness.