Proofreading used to be done by humans, who would mark up pages of manuscript with weird and arcane marks.
These days, lots of people rely on the spellcheckers that come with whatever wordprocessing package is on their computer. There’s a well-known poem which highlights the pitfalls of over-reliance on spell checkers.
And here’s Taylor Mali performing at the Bowery Poetry Club back in 2005, in a piece calle “The the impotence of proofreading“. It’s harder to follow the spoken word than the written word, so you may have to watch it more than once!

This quack paste lights up my wok and makes me smell, without a debt.
This pose maid my stomache hurd. I enjoined it allot.