In The Help what did Minny do to Hilly that she said she could never tell anyone when she was fired?

When Minny was working for Hilly's mother's, Hilly treated
Minny poorly and always accused her of trying to steal things from the home.  Minny was
fed-up with Hilly's disrespectful attitude, so she made a chocolate mousse pie and put
her own excrement into the mousse.  Minny would not let Hilly's mother eat any of the
pie, and insisted that it was only for Hilly to enjoy.  Hilly ate it up.  Minny was
initially excited about her revenge, but afterwards, she realized that her actions could
have severe consequences.  No one will want to hire Minny as a maid if word gets out
about what she did, and Minny fears that she will be left without a job.  But Hilly
cannot stand for anyone to know this because she too will be socially embarrassed having
to admit that she ate the pie.