1. Memory rated for higher voltage might not run at lower voltage, e.g. 2.1V RAM at 1.8V. The system might not start. The lower voltage RAM will most likely get damaged if voltage is increased.
2. Memory rated for higher frequency, e.g. 800MHz, might run at lower frequency, e.g. 667MHz, but it is not guaranteed that it will run error-free.
3. Memory with relaxed timings, but same frequency, ex. 5-5-5-15 will not run at ex. 4-4-4-12 in most cases.