From: Richard Fateman on
In your Mathematica documentation reader, look for

tutorial/WhyYouDoNotUsuallyNeedToKnowAboutInternals

and scroll down to In[9] ...

and notice that the example fails because it is in notebook form. :)


I was inspired to write out a more detailed commentary on this document,
which you can see at

http://www.cs.berkeley.edu/~fateman/papers/MmaInternalsResponse.pdf

no notebook reader required.