Undocumented Postgres error: "failed to fetch old tuple for AFTER trigger"

I'm running a very large series of commands - mainly DDL but some DML as
well - in a large transaction. I get the following error, which doesn't
seem to be documented:

"ERROR: failed to fetch old tuple for AFTER trigger
: COMMIT"

There are no triggers that I'm aware of. I've gotten this error when
running the transaction on two different instances of the database. I also
did a VACUUM FULL, which didn't help.

Breaking down the commands into a series of about 7 smaller transactions
works fine. Each transaction runs totally fine by itself. But, when I run
them all in one giant transaction, I get errors.

I'm running Postgres 8.2.1 on Windows XP. If anyone can help, I'd be very
greatful.