Hello!
I've had a coffee stain issue on my M8, some time at the start of the last year.
I didn't notice any discussions on the internet at that time and, since it didn't show up in my images, I forgot about it.
I actually think I bought it from my friend back then with the coffee stain present. I thought he just accidentally pressed something against the screen, while carrying it in a bag, but I didn't really care since the actual images were just fine.
Anyway, after a few months I noticed that the stain just dissapeared. It no longer shows up on the screen and the camera runs just fine, no problems to report here.
Hope this helped, somehow!