I'm going to see if I can reproduce it below (keeps happening when I comment on threads, but I don't want to hijack those threads to show it). We'll see if Murphy's Law allows it!
Basically, about 50% of the time the Topic comment box forces my text to go vertical instead of horizontal. I've seen this on both Safari and Firefox. To fix it, I have to discard the comment and start over. Most of the time it starts when I hit the backspace or delete button - it deletes everything and begins the vertical scroll. And then sometimes it just happens on its own.