On this page:
http://en.wikipedia.org/wiki/Zone_plate
I'm getting "Failed to parse (Can't write to or create math output
directory): r_{n} = \sqrt{n f \lambda}" in place of the mathtex
equations.
However:
When I click "edit", the equations display correctly in the preview.
I'm guessing purging the cache or something might fix it, but in case
no one has seen this before I thought I'd report it.
Steve