Stealth randomly turns off, people can see through stealth and improved vanish does nothing sometimes.
All known bugs. Touch of the Grave proccing also pulls you out of stealth. There are also some spots out in the world that pull you out of stealth just by walking past them.
But Blizzard never does anything about stealth bugs.
Have been having this issue aswell and ended up making an addon that darkens the bottom of my screen when im in stealth. like limb darkening. because i would run around like a dum dum still thinking i was in stealth both in pve and pvp and my eyes are not on my character to see if im still stealthed or not 
They will never fix this bug. This has been a bug for years. People have complained about it for years. They just do not care about the rogue class; which is why nobody plays it.