These make me crabby because I don't always type the right thing. And I get to repeat the gobbledygook typing exercise. Sometimes I get it wrong because of glare on the screen. Sometimes it's because of an odd font, a weird color, a really groovy ripple, or a really spotty background texture. Or I fat-finger in the text. Sometimes, it's because I am in the bifocal club, and I think that adds to my annoyance.
But surely it can't be just my eyesight. I pass the eyecharts at the doc's, the Air Force checked to make certain I could tell the difference between fuchsia and avocado, and they let me keep my license down at the driver's bureau. I think that sometimes, they are just tough to read.