- Visit a question, read the answers.
- Click on a link to some other page.
- Return to the question page using the browser's back button.
- Try to vote on an answer.
- Reload the page.
- See that the vote was cast for the wrong answer.
If the answers were reordered when returning to the page, there's a high chance the vote will be cast on the wrong answer. Say in step 1 the answers were in order A, B, C but were reordered to C, A, B (or an additional answer D appeared in the meantime) when returning in step 3; when voting on answer A the vote will be cast on answer C, which is apparent when reloading the page.
Confirmed on Safari OS X (for a number of past versions, this has existed for a while).
Some more details, as it just happened on this question:
I had downvoted the question, then clicked through to another page. I returned to the question and two new answers were posted in the meantime. The first answer was being shown as being voted down by me (false). The markup looked like this:
Notice it has both
I then tried to explicitly vote on the answer, which gave me the error that I "had already voted on the question" and unless it changes I couldn't vote again etc etc. Here's the AJAX request: