A couple of times now, I've been entering a large matrix here or on math.SE and, in the preview window, all the rows appear on a single line. Once the question posts, it displays correctly. I most recently ran into this bug on this question.
I'm afraid this is somewhat of a Heisenbug: I just tried to reproduce by (1) editing my question, (2) copy-pasting my equation into a new ask box and (3) manually copy-typing the matrix into a new ask box, and none of them reproduced the bug. It has happened to me a few times now, though.
Has anyone else seen this? Does anyone have a reliable way to reproduce it?