WaitOnLock error - what does this mean?

I got the error:

ERROR: WaitOnLock: error on wakeup - Aborting this transaction

Can anyone explain what this means?

I am testing my database by having it do inserts continuously using a bash
while true loop that calls psql --file to do the inserts.  When I try to do a
different kind of insert from a php script at the same time, I get the above
error.  I can provide more details if needed.  One thing, I have PL/pgSQL
triggers that cause a chain of updates to happen for each insert.

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            - Robert