I'll answer my own, for the benefit of those who follow...
I -think- what happened in this case was that an Eclipse .classpath
that had a bad reference to javax.servlet.jar was handed-out. The
result was that the project wouldn't build until Eclipse was happy
with the all library entries. Someone decided to make Eclipse happy
by finding an old copy of javax.servlet.jar and placing it where the
.classpath file said it should go... right smack where an old copy of
the jar file didn't belong! The true solution would have been to
simply remove the entry from the .classpath file, and to never have copied the jar file into the container directory structure. Once that earlier work had been reversed, it worked like a charm.