Query failed: ERROR: deadlock detected

PostgreSQL 8.0.3: Any idea what this is?

---
Warning: pg_query(): Query failed: ERROR:  deadlock detected
DETAIL:  Process 8835 waits for ShareLock on transaction 193588236;
blocked by process 8834.
Process 8834 waits for ShareLock on transaction 193588228; blocked by
process 8835
---

I have never seen such a log entry until now.