This article is from the FAQ, by with numerous contributions by others.
The INS can refuse entry (which functionally amounts to the
same thing as canceling a visa). That happens mostly when
the INS discovers something that is contrary to your declared
intention in getting the B-1/B-2.
Typical example: you have a fiance(e) in the U.S. INS discovers
love letters (long, mushy ones) in your documents. Or during
conversation with an INS officer you let slip that you are panning
to get married in the U.S. These are some of the examples I have
seen personally. Legally, there is a bunch of grounds such as past
criminal convictions, past immigration violations, certain diseases etc.
(technically called "grounds of exclusion") that can bar ones entry
despite possession of a valid visa.