Memory questions

I heard somewhere that mixing your memory is usually a bad idea, also even using different kits say 2x4gb and separate 2x4gb is not good either. How true is this?

I currently have a G.Skill 1600MHz 4gb (http://www.gskill.com/en/product/f3-12800cl9d-8gbxl) mem stick that when running through memtest x86 gives me a ton of errors (not overclocked, all is stock) they have a lifelong warranty, but the require both sticks for testing so I haven't gotten into it yet. In the end, is mixing the same memory very bad, I can understand mixing different manufacturers of memory would be, but how about the same manufacturer and the same speeds?

as long as you get the exact same model of memory you should be fine, I've mixed 2 HyperX Fury kits from kingston and they are fine :)