Everyone makes typographical errors. Most get caught, usually while the person is typing. Some get caught when (if) a person looks over what they've written before posting. However, some words slip through easier than others. This is especially true when the error is actually a word in its own right. "Sipped" instead of "slipped" will get missed much more often than "slippeed", for example. The writer knows what they intended to type, and nothing is glaringly out of place, so the brain essentially fools itself.
Auto-correct and auto-spellcheck can also fool the brain.
All of this is why most writers are horrible at self-editing, unless they allow enough time to pass that what they've written has been flushed from their short-term memory.
Some errors are also really hard to spot in many fonts, with the "r" being a top culprit.
Of course, some people really are clueless. I once had a middle-school kid tell me that he loved "grazed" donuts.
