Comparators output different redstone signal strengths based on either the power level of the block behind them, or the fullness of the container behind them. The container can be directly behind, or two blocks behind if there is a solid block in-between. Comparators prioritize reading containers--in other words, whenever it is reading a container it ignores the power level of the block behind it. However, when a block is moved by a piston or placed by a player such that a container and a powered block both come in range of a comparator at the same time, then the comparator may briefly output the power level before switching its output to the reflect container fullness. With blocks moved by pistons it always outputs the power level first, but with blocks placed by players there is a 50/50 chance that it will output the power level before the container fullness.

Steps to reproduce

1. Place a detector rail diagonally behind a comparator, then summon an empty hopper/chest-minecart on the detector rail.
2. Use a piston to push the detector rail directly behind the comparator.
3. Place an empty barrel diagonally behind a comparator.
4. Two blocks directly behind the comparator in step (3), place a repeater facing toward the comparator and a redstone torch behind the repeater.
5. Use a piston to push the barrel directly behind the comparator (between the repeater and the comparator).
6. Place a repeater diagonally behind a comparator, facing toward the space directly behind the comparator. Place a redstone torch behind the repeater.
7. Place an empty barrel two blocks behind the comparator in step (6).
8. Repeatedly place and break solid blocks in the space between the barrel and the comparator.

Expected results

In steps (2), (5), and (8), the comparator should not output a redstone signal because it is reading empty containers.

Observed results

In steps (2) and (5) the comparator flashes a redstone signal briefly. In step (8) it flashes a redstone signal 50% of the time.