If you look at the following questions tagged sml, ocaml and F# respectively, you'll see that the formatting is broken:
It seems the problem is that
'foo is interpreted as the start of a string, not as a type variable as it should be.
At first I thought that the question were not correctly interpreted as ml-questions, but looking at the source code shows me that the
<code> tags do indeed have class
So I looked around at the prettify homepage and found this bug, which seems to describe the same problem. It is marked as fixed.
Since the bug is still happening on SO, I suspect SO might be using an old version of lang-ml.js (or the bug isn't really fixed - but it looks fixed in the source), so I suggest upgrading to the current version.