NOTICE: (transaction aborted): queries ignored until END

Hi,

I'm use Postgres-6.5.1 on Linux. My database contains two tables and two
indices. I wrote simple program on C that
- begins transaction:  BEGIN WORK
- inserts many (10-500) rows in the first table
- commits the transaction: COMMIT
- begins transaction: BEGIN WORK
- inserts many (1-50) rows in the second table
- commits the transaction: COMMIT

During execution of this program I receive very often the following
message:

                NOTICE:  (transaction aborted): queries ignored until
END

that repeated many times.

What means this message? And why this happens?

Best regards,
Alex