tuple concurrently updated

One of my machines has two CPUs, and in some cases I build a pair
of indexes in parallel to take advantage of this.  (I can't seem
to do an ALTER TABLE ADD PRIMARY KEY in parallel with an index
build, though.) Recently, though, I received the message "ERROR:
simple_heap_update: tuple concurrently updated." Can anybody tell
me more about this error and its consequences?

cjs
-- 
Curt Sampson  <cjs@cynic.net>   +81 90 7737 2974   http://www.netbsd.org   Don't you know, in this new Dark Age, we're
alllight.  --XTC