Tweak any PGF function to have a side effect, for example edit pgfmath.code.tex.ltxml as follows:

    e          => sub { NoteSTDERR('evaluating `e`'); $E; },

Then run the innocuous

\pgfmathparse{e}\pgfmathresult

LaTeXML will print 'evaluating e' twice!

Aside from the performance impact, which is probably negligible, this is making it impossible to replicate PGF's random number generator because rand gets called twice every time.