There is always a risk of fracture with porcelain.  It should not break
in normal use, but the errant olive pit, the patient with habits, etc.
may put a hairline crack into it, and it may chip off then or some
indeterminate time later.  Cleanliness of the lab and well-controlled
baking at the lab are necessary.
    The tooth is still covered by metal, so there is no need to act
immediately.  Usually the problem is if the porcelain fractures between
the teeth, in which case food impaction can become a problem.  Often
though if it is the side of the tooth any rough edges can be smoothed
off and the crown retained indefinitely (assuming there is no cosmetic
problem).

Steve

I have a crown (porcelain over metal) on a back molar on the top.  It
was put on four years ago, while I was pregnant.  Recently, the back
part of it broke off.  I am not experiencing any pain or discomfort.
I'm wondering if it is common for crowns to break so soon.  Do I have
to have it replaced if the tooth is not exposed?  It was very costly
when I had it put on and our dental insurance does not cover crowns.  I
really don't want to have to replace it if at all possible.  Should I
be responsible for the replacement cost or is there some type of
warranty on them?  Before I make an appointment to be seen, I'd like to
know if it "should" have broken after four years and if I should be
responsible for all of the costs involved.

Thank you in advance for any information you can provide.