I up-voted an answer, and then someone left a comment proving that the answer was in fact incorrect, so I redacted my vote. I was actually a bit surprised that I was allowed to because it had been awhile, and we all know how nit-picky SO is about these things. Later I refreshed the page and noticed he had amended his answer and wanted to up-vote him again. Not allowed it says!
I'm guessing what happened is that when I went to redact my up-vote he had already edited his answer, but I didn't refresh the page, so I didn't actually see his edit. So my redact counted as a vote on the newly edited answer, which nullified my future vote.