Ionization, radiation, transistor wear, color filter aging and so on... In normal use you will never see anything weird except dead pixel count slowly rising that are not mapped out. The worst ones are those that are a little bit lighter/darker than the others. Hard to map them out if they are not exactly stuck. Since I am on the phone this link may be an interesting readhttp://www2.ensc.sfu.ca/~glennc/apspapers.html
Basically all electronic devices degrade. Take your cheap memory card for example. Or Solid state drives and so on. Everything fails. Just depends on the time. I'll try to find the PDF materials regarding Cmos sensors used in medicine where they had problems. In the end the sensor is analog device and quite many things can mess up the analog signal on nano scale circuit.
