Minecraft 1.13-pre6, Minecraft 1.13-pre7, Minecraft 1.13-pre8, Minecraft 1.13-pre10, Minecraft 1.13, Minecraft 18w30a, Minecraft 18w31a
If you build the circuit shown in picture 1 and activate the observer, the piston extends, as expected, only once. But if you place a pushable block in front of the piston, it extends twice as if it was powered with two short pulses. This didn´t happen before prerelease 6. The expected behavior would be that the piston in this case also only extends once, because it shouldn't make a difference whether there is a block in front of the piston or not.
The same thing happens with sticky pistons.
Another problem with chaining repeaters is shown in picture 2. If you build the circuit and activate the observer in prerelease 6, the sticky piston drops its block. In versions before pre6, the piston wouldn`t drop its block. This is the expected behavior, because the piston should be powered for 2 redstone ticks, meaning it can extend and retract without dropping the block in front of it.
Picture 3 has the same repeater configuration as picture 2, but with two sticky pistons stacked on top of eachother. If you activate the circuit, you notice that the blocks in front of the pistons swap their positions. If you look closely at the pistons, you can see that the piston, which has a block directly in front of it when you activate the circuit finishes its extension before the other piston. This means that the piston detects a shorter pulse when it has a block directly in front of it than when it hasn't. This definitely shouldn't happen and both pistons should detect the same pulse length regardless of whether they have a block in front of themself or not