A clock (or compass) swings around whenever you suddenly look at it.

Steps to reproduce:
1. Look at a clock,
2. Look away from all clocks: turn so you can't see any wall clocks, close all inventories with clocks in them – this even works if the only clock is in your hand, and you simply hide the GUI with the F1 key.
3. Wait several minutes,
4. Look at any clock (turn back to the wall clock, or toggle F1 to view your held clock, or open that inventory to view a clock there) – and notice the clock moving.

The same goes for compasses:
1. Look at a compass,
2. Stow them in an inventory, or hide the GUI, so you can't see any compasses,
3. Turn to some other angle,
4. Look at the compass again, and see it swing to the correct position.

Doesn't happen in 1.8.8, but does as of the first 1.9 snapshot.

Maybe related to MC-83905, compass spinning when player faces south,
Or MC-88103, clock spinning at noon.