That being said, there are reasons as to why we are sometimes prone to making typing errors:
When we proofread something we've written, we are aware of the meaning that we wish to convey. Expecting the meaning to be present makes it easier to miss aspects of the text that are absent.
Someone who is reading something we've typed is more likely to notice the errors. The other person's brain is reading the text for the very first time, so there is more attention paid to details without having a prior knowledge of what the text is meant to convey.