I doubt if you would find many punctuation or spelling mistakes in a book that you have been reading. That is because a publisher reads through every piece of work thoroughly, checking for mistakes before it is finally printed.
The same is true of newspaper articles, although daily publications have to work within much tighter deadlines. Therefore, mistakes are more likely to slip through. The people who try to make sure that no mistakes appear in books, newspapers or magazines are called proofreaders.
I am sure that you had the experience of handing in a piece of work that you thought was really good - only to receive it back from your teacher covered in red ink. This situation is annoying to both teacher and student. The teacher thinks that you have just been lazy and may write this comment: 'This is careless work - check more carefully for mistakes.' The student feels annoyed because he/she did check carefully, but just did not spot the errors.
Often the situation is even more annoying because once the errors are pointed out to you, you see the problem immediately and understand why you are wrong. So why did you not spot those mistakes in the first place?
The answer is simple. It is very difficult to spot our own errors. We have invested a lot of thought and effort into what we have written. It is very hard to admit to ourselves that it might be wrong or could be improved. Even spellcheckers do not always help. If you write 'bough' instead of 'bow', then as far as a spellcheck is concerned, this is correct spelling. It will not tell you that you have used the wrong homophone. The answer is to develop a systematic way which will help you to spot your mistakes.
First, spelling. When we read, our eye tends to skip along the line picking up the overall meaning of what we are reading. We do not read every word laboriously. This is especially true when we read something we have written ourselves, as we already know what it says. As our eye skips ever faster along the line, we miss the obvious mistakes.
One solution is to read backwards. Start from the end and work your way towards the beginning of the line. Whenever you come to a word whose spelling you are not quite sure about, underline it with a pencil. When you have finished checking the whole piece, you can look up all the words that you have underlined in a dictionary.