Pistons can only be re-pulsed 5 or more gameticks later.

All redstone components can become stuck in a powered state when indirectly unpowered

When a sticky piston ejects an observer into another observer the 2 output pulses are too close together

Under certain conditions pistons can't be powered for the first 2 gameticks after being moved

Inconsistency with update order when pistons retract other pistons

Observers don't detect retracting sticky pistons

Observers detect moved blocks late in 1.13