I wanted to upvote this answer and so I did. However, half an hour later the upvote was registered to this answer. I upvoted the intended answer again. Then this appeared, suggesting I upvoted yet another answer.
This time I didn't do anything and after a while, coming back, I got this.
I've encountered this problem before. I'm using Safari 6.0.3 (on a Mac with OS X 10.8.3) and have AdBlock 2.5.61 installed.