Hardly noticeable but I still decided to report it - just noticed: at Find the number of boolean functions of n variable that satisfy the following condition one sees this
The second $k$ below is obviously spurious...
PS Added the link as suggested by quid; the bug is not always there however, right now it reappeared after I reloaded that page.
PPS On Windows 10 pro I see this in Waterfox 44.0.3 and Chrome 48.0.2564.116 m (64-bit), have not tried any other browsers
Later - seen another instance of the phenomenon in the "Related" column of Is this a model for $K$-theory of a triangulated category? which might shed some light on the nature of it. Result looks like
while the actual content of the garbled part should be "Proof that the homotopy category of a stable $\infty$-category is triangulated"
Thus the whole thing is broken into columns in a wrong way somehow.
Also as pointed out by Martin Sleziak in a comment below this also happens on math.SE and physics.SE. At links provided by him there is a suggested solution by Davide Cervone who seems to be one of the developers of MathJax