Containers with items will trigger observers when a world is loaded.

In minecraft, an observer should only triggered when a block is updated.  Because of this bug, observers can be triggered when a minecraft world is loaded.

The bug can happen if the player places a container(e.g barrel) filled with items on the face of an observer. When the world is then reloaded, the observer will trigger.

This can cause issues for single-use redstone contraptions.