d_n+1 is highlighted as the variable
d_n, operator+, and 1. This is incorrect; the symbol should be all one color.
First, make sure the correct language highlighter is being used.
If the question is tagged lisp that should be correct, but verify that the code class is correct in Google Chrome inspector.
If this is not the case, try to fix the tags first and remember diamond mods can change the prettify language per-tag on the tag page.
<!-- language: lang-or-tag-here --> code goes here
It's very important you have the linebreak there.
If you've done this and the highlighting is still weird, indeed it must be filed as a bug against Google Code Prettify.