If you select from a table and insert the results into the same table, the query never returns, provided the result set is sufficiently large.

To reproduce:

CREATE TABLE t (x INT);
INSERT INTO t SELECT generate_series(1, 25000);
INSERT INTO t SELECT * FROM t;

The last query never completes. Based on the logs, it seems like this happens when the insert is large enough to trigger a range split, and then it causes an endless cycle of range splits. The splits were a symptom, not the cause. See discussion below.

This issue is present in both 2.0 and 2.1.