From A. O. Scott’s review of The Imitation Game, the new Alan Turing biopic:
“The Imitation Game” is rated PG-13 (Parents strongly cautioned). Illicit sex, cataclysmic violence and advanced math, most of it mentioned rather than shown.
First, advanced math is reason for caution? Um, no.
Second, what’s the difference between “mentioning” and “showing” mathematics anyway? This seems to be connected to the philosophical use-mention distinction, Godel numbering, and so on, but I doubt that Scott is trying to make a metamathematical point. This may just mean that people in the movie don’t sit around scribbling on pieces of paper but rather talk about the fact that they have done so, which is good, because I’ve watched people do math and there’s really nothing to see.
God plays dice 
I’m pretty sure Scott was making a joke, along with the violence and sex, in that there’s really little to caution people about.
As for mentioning versus showing mathematics, nobody ever really proves a theorem, or even explains their reasoning. There’s the idea that math is important to building their bombe, but not really an explanation of why.
I’d imagine that to be the difference between mentioning and showing mathematics. It’s one thing to have characters (say) mention calculus, but another thing to have the reasoning behind something mathematical be discussed or even be relevant to the plot, even so much as (say) Nash equilibriums were part of A Beautiful Mind.