The typo in a message is nearly always easier to see a second after you send it. Before that, you were reading what you meant, which is a very forgiving text: your eye slides straight over “teh” because you know perfectly well what goes there. Pressing send changes your job. You stop being the person who wrote it and become, for the first time, somebody reading it, with no say over what it says, which is exactly the reader the typo was waiting for. Careful people get the same effect by leaving a draft overnight, or reading it aloud, or going through it backwards. But the cheapest way anyone has found to read your own words like a stranger is to make them impossible to change. It is probably the only proofreading method that works every time, and it only works too late.