It's not even just same brand and model, when you have multiple sticks they should be coming from a kit where the manufacturer is guaranteeing that those particular modules work together.
To put numbers on these things, you can mix sticks about 97% of the time.
When you mix multiple sticks you are restricted to the slower of the two. For example, combining DDR3-1866 10-10-10 with DDR3-1600 9-9-9 we are only guaranteed that *if they work together at all* it would be at 1600MT/s 10-10-10. Now it may be that the 1866MT/s module will run at 1600MT/s 9-9-9; it may be that it's also rated for 10-10-10 at 1600MT/s. It may be that the 1600MT/s module will run at 1866MT/s 10-10-10 but either way those other settings are memory overclocks. In this contrived example, it would either never work or would safely run at 1600MT/s 10-10-10 and *might* support other speeds but they are overclocks for one of the modules.