It happens most of the time when you are on a server with loads of plugins OR if you have a really slow computer. You should do multiple tests to see when it happens.
Does it happens only on servers or in a solo world?
An image worth 1000 words. So I've created a .gif!
I've built a wall of withe stained glass with a floor of orange stained glass. It can be clearly seen that there is a "glitch" between the withe and the orange stained clay
It's still concern the current Minecraft version 1.8.8
The bug happen on stained glass
I confirm