There’s a common perception that it’s harder to proofread a text on a computer screen than in print. That certainly matches my perceptions. I’m not sure why it would be, though. One thought is just that when you’ve actually got a printed copy in your hand, you know you’re about to send it off to wherever it’s going, and so you check it more carefully. But is that all there is to it? It sure seems to me that I’ve missed glaring typos on screens that simply jumped out at me when the text was printed.
That last word was supposed to be “experience”. That’s not a typo, though, that’s simply a non-functioning brain.
I can’t answer why, but I concur with the perception.
It isn’t limited to words, either. I work with CAD on a daily basis, and errors are much more obvious to me when they are sent to a large plotter.
A looong time ago I read something that was along the lines of ‘why do people print out things to read them instead of reading them on the computer screen’, so if the theory was that your eyes focus differently on a hard copy. With a CRT you have basically a flickering image which is harder to focus on. You tend to ‘skim’ more and do more ‘predictive processing’ where your mind is filling in the missed information. If so then it would seem like spelling errors are more likely.
I would like to know if this applies to LCD monitors as well.
Even the cheapest inkjet printouts have a far higher resolution than a computer screen. I suspect this difference can have an effect, when the ability to pick out errors is dependent on recognition of word & letter shapes and patterns.
I have a WAG to add to that: there seems to be something to the idea that relected light (like from a hard copy) is easier on the eyes than … er … non-reflected light (dammit, what’s the word for that?). Tired eyes from looking at a screen = skipped errors, due to ‘predictive processing’, I’d bet.
FWIW, when I used to work for my college newspaper, the copy editors edited the stories on a computer screen. However, the screens were black with the words in gray (the same screens at our city newspaper, at the time, had black screens with green words). This was on DOS machines, so there was no Windows/Mac style clutter to distract the eyes, either.
I recently switched to LCD, and yup, same deal.
Thanks, Excalibre, for starting this thread, as I thought all these years it was just me. 
I think the explanations given are part of it, but also wonder if it also may not partially be due to the fact that we are more in a hurry while at a computer, just due to the fact that they are quicker at writing, so we tend to do things faster (and thus less accurately)?
I don’t even like to read extensive copy on the screen. If a user’s manual or news story is too long, I print it out as can glean more from it on the printed page.
Now, I Previewed this, but will not be surprised if tthere is a typo or mispelled word here somewhere!
As someone who has spent many decades staring at everything from TTYs to orange monochrome CRTs to LCDs, the issue isn’t format.
I easily spot other people’s typos on the SDMB. But spotting my own without preview is hard. Even with preview quite a few get thru. (So I cut other people slack, usually.) A few days ago, it was about my third read thru a thread I had posted to that I noticed a typo in my post.
Putting a document is very different format helps a lot. Your eyes forget how it was originally formatted and take it all in again.
I think it’s this disconnect between “what you typed” and “what you though you typed” that causes most problems. I have look over the shoulders of a lot of CS students in labs and know this is extremely common. (As well as in my own programming.) I once went 5 minutes with one student asking her over and over to to look at the screen and tell me exactly what she typed. And she kept telling me over and over what she thought she typed. (It is crucial in CS to be self-critical of ones own work. Having someone else point out the extra “.” doesn’t help.)
Preview is your friend, but friends aren’t perfect.
I usually catch my typos as I make them. I am a touch typist and feel a typo when it is made. Perhaps those who are hunt and peck typists make more typos that they don’t catch. I have seen two finger typists who type very fast, but make many mistakes.
The problem for decades was the white print on black background, which just looks wrong.
It’s still a problem for people who work with mainframes and terminals, where many systems still have white on black and white on blue.
Pssst…misspelled.
Interestingly, I hit reply in order to point out the misspelling of misspelled, but once I saw the quote in the box, I also noticed the double ‘t’ in “there”, which I had initially missed. I think this is an example of a slightly different format allowing your brain to reset and catch errors.
I put out a Friday report which I’m working on now which I always type first in Word. I proofread it both onscreen and in a printed copy, but when I copy it into an email to send it off, I inevitably find additional errors.
Hey, I did that on purpose, just to see if y’all were on your toes. Or is that teos? 
I agree that the differences between a computer monitor and paper are important, but there must be other factors. There’s almost no difference between you previewing your own post and how it appears after you’ve submitted it, but how many times have you seen typos only after submitting? That’s the reason people keep asking about being able to edit their own posts (although the risks involved outweigh the benefits).
For me, at least, an even greater help would be a good spell checker here!
I also wonder whether it is the horizontal position of the document to be checked, and/or the mindset of the checker.
I never hold a sheet of paper or a large format drawing when checking it. I put it down on a table or desktop under good light, and then examine it with a marking pen in hand. So, my head is inclined, and I am there to correct rather than compose. Mistakes stand out in a way they never do on the screen.