I noticed yesterday that one of my Dell 2001FP's at work had a horizontal stain on the face. About 4" wide and 1/4" tall. Sharp edged on top and fading as it falls.

Won't come clean. I kept racking my brain, how could it happen, when I noticed a pad of post-its laying on the table. Exact match!

There is no residue that I can feel, or that removes with any benign cleaners. Rats.
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Glenn