I have noticed this behavior in this post. Here is an example:
$|s_nt_n - st| = |s_nt_n - s_nt + s_nt -st| \le |s_nt_n - s_nt| + |s_nt - st| = |s_n|*|t_n - t| + |t|*|s_n -s|$.
It behaves the same way if the comma is replaced by some actual text (since the comments indicated different behavior for different uesr, I've added several samples with various lengths of the word immediately following the formula):
$|s_nt_n - st| = |s_nt_n - s_nt + s_nt -st| \le |s_nt_n - s_nt| + |s_nt - st| = |s_n|*|t_n - t| + |t|*|s_n -s|$ test
$|s_nt_n - st| = |s_nt_n - s_nt + s_nt -st| \le |s_nt_n - s_nt| + |s_nt - st| = |s_n|*|t_n - t| + |t|*|s_n -s|$ tes
$|s_nt_n - st| = |s_nt_n - s_nt + s_nt -st| \le |s_nt_n - s_nt| + |s_nt - st| = |s_n|*|t_n - t| + |t|*|s_n -s|$ te
$|s_nt_n - st| = |s_nt_n - s_nt + s_nt -st| \le |s_nt_n - s_nt| + |s_nt - st| = |s_n|*|t_n - t| + |t|*|s_n -s|$ t
$|s_nt_n - st| = |s_nt_n - s_nt + s_nt -st| \le |s_nt_n - s_nt| + |s_nt - st| = |s_n|*|t_n - t| + |t|*|s_n -s|$ testtest
The problem is that MathJax is somehow divided into two lines and the text looks as a part of the formula. It might be the same problem as described here, although that post discussed specifically titles in the profile page.
Here is how this post looks on my computer.
I specifically speak about he behavior at the linebreak: