There was a recent automated testing failure from test/checkpoint, with the following error message:

[1499309768:383511][2122:0x7fb2c2aec700], t, WT_SESSION.checkpoint: read timestamp 11732 older than oldest timestamp: Invalid argument
t: session.checkpoint: Invalid argument

Upon inspection of test/checkpoint I don't see anything that stops the worker thread calling set_timestamp(.., "oldest_timestamp=XX") with a timestamp newer than that about to be used to create a checkpoint. It's made unlikely by only updating the oldest timestamp once every 100 allocated timestamps, but that isn't good enough for automated testing reliability.