unstable

Cancer cells are inherently unstable, and so they can continue to mutate- some of them faster than their buddies.

Waves are probability, and they're unstable if you start to get electrons and they analyze them at the level of an atom.

But, excavating a tunnel underground in unstable material can lead to some dangerous situations, like the 2010 mining accident in Chile when 33 men were trapped deep in subsurface for more than 2 months.

This means that a constant battle of seepage flowing in from the bottom of the caisson, or much worse, unstable soils that can rapidly erode and allow the caisson to flood.