The very first thing in this answer to a Ruby question is a code block, but it's displaying as prose. But if you click "edit" to fix it, the code block displays properly in the preview pane!
It appears to be formatted properly as a code block (indented 4 spaces) and the indent uses regular old space characters ($20). There aren't any backticks or other unusual characters in it. Why is this not displaying as code?
[All the other "why doesn't this code display properly" answers I could find in meta have to do with code blocks appearing after a list (which isn't the case here).]