A bug or an iOS8 accessibility feature?

Hi folks,

So this morning I must've slipped my 5S in my pocket without locking the screen. When I pulled my phone out again, this is what I see on my screen ....

The weird thing is that the black & white panel was moveable, I could drag it up and down the screen using the small white bar at the bottom of the panel.

I couldn't work out how to get rid of it so powered off and on again and it was gone. I've not been able to replicate since .... some kind of accessibility function?

Any ideas .... ?