Screen display
I've just experienced a rather odd thing on my iMac (late 2013).
It's hard to explain, but it's like everything was overlaying each other, with elongated black triangles and while I could see everything, I couldn't do anything. I tried taking a screenshot, but it doesn't reflect what I actually saw. I couldn't restart and had to turn off via the power button at the back.
Does this mean my iMac is its way out? :o(