I have a late 2009 iMac 21". I recently noticed what I thought was a water stain on the screen, but then I realised it wasn't on the surface of the glass, so I got interested.

There's what looks like a water stain, and a verticle line of sorts - but: it doesn't show when the screen is white, or when pure RGB colours are displayed. It only shows for anything with greyscale, or CMYK colours. So I notice it most when watching films, where the colours are variable.

In the following screenshots, you have white, RGB pure yellow/red/blue, and lastly 50% grey (where the blemish and line are obvious towards the left of the screen). It barely shows using CMYK yellow, but is more obvious in CMYK magenta or cyan (I didn't get photos of those, since the grey tells the story).

Something's definitely rotten, I must get it looked at.

If it's brokenless, don't suffix it...