A few days ago, I was working on an Excel spreadsheet and I noticed two cells had orange vertical marks in the cells. They looked like something was being flagged. I tried emptying the contents of the cells, but the marks didn't go away. Later, I was trying to print some medical test results, and noticed that the same marks were in the priint preview screen, but when I printed the test, the orange marks didn't print.
I started experimenting and noticed the marks appeared on some programs, but not all. I Googled the problem, and didn't get any satisfactory suggestions. I rebooted, but that didn't help.
I wondered when the marks first showed up, so I went upstairs and asked my wife if she had noticed them. She looked at the screen, thought for a minute, licked her finger, and rubbed out the marks.
Color me embarrassed.