This SE-promoting ad, that managed to cause some problems in the past. (Now redesigned a bit).
Sometimes it displays a question title from math.stackexchange that includes a mathjax expression. In this case, the expression is shown as mere text. Confused me a bit when I saw it first time.
Would be better to either render the actual equation for that ad (would be much more appealing, wouldn't it?) or ban questions with mathjax in their titles from appearing.