Well, I noticed this bug always appears on the first post, and I couldn't help wonder why - removing the border fixed it, but it still appears for me.
See it? It's on the left of the border of the Report button (so, the border by the border).
The first image was a screenshot taken on these project forums, and it appears on all Cranefly based themes, unless they've removed the border on the left of the Report button. But I'm wondering if there is another solution.
It only appears on the topic starter, too.