In waterstreamer's recent post, they used a table to present the possible Jordan normal forms of a $3\times3$ matrix. As they comment, and I checked, the table looks just fine on preview, but terrible on post:

Preview:

Post:

Why does it behave that way?

EDIT: A new revision removes :s in the subheader line, and it now renders fine. So that explains how to get a correct rendering on post. But why did it work on preview before, if that's not the right syntax?