Well the whole change in texture thing explains what happened to my chapstick many years ago. Suddenly it stopped hydrating and left a gross chalky layer on my lips!

I think as long as you pay attention to general hygiene - clean brushes/buds, no fingers, select pump bottles over jars and store everything properly, you should be okay.

Once an eyeshadow stops delivering as much pigment, it's usually time to go. Another thing to think about is; you either use an eyeshadow a lot, often or only sometimes. After two years, it's either gone, halfway there or barely touched. Halfway there is when you should be watching it. If you use it a lot, you probably need to replace it anyway. Barely touched - give it more stage time or rethink why you got it.