The number of embarrassing blunders spellcheck has saved us from as a species is likely very high, but it's a double-edged sword. Where you'd once ask a proofreader (hello) to check your writing, or at the very least a literate friend, now you can run a spellcheck and feel like you've weeded out all your potentially humiliating errors with a click.
I'm not maligning spellcheck, because it's a helpful tool. I'm often grateful for it myself. But there are limits to spellcheck's abilities that make it a poor substitute for professional eyes. The above is a perfect example of this. Each of the highlighted words here is wrong - oh god, so wrong! - but not one of them will have been picked up by spellcheck. Why? Because they're spelt perfectly! The mix-up here is not an issue of spelling, but of vocabulary itself. Spellcheck is rendered supremely useless.
The word isle here should be aisle; alter should be altar; and peaked should be peeked. It's easy to see where the confusion has come from, because all of these are homophones.
Phonetics. a word pronounced the same as another but differing in meaning, whether spelled the same way or not, as heir and air.
If you're into your very specific linguistic terminology, these are all also heterographs, meaning they are spelt differently despite sounding the same. Another fun related term is an oronym, which refers to whole phrases that sound the same, such as 'euthanasia' vs 'youth in Asia'. There is a fair bit of overlap between homophones, oronyms, malapropisms, mondegreens, and eggcorns, where the main distinction seems to be the context and intent behind their use.
In any case, the moral of the story is that spellcheck is only reliable up to a point, and as we learnt from the IT Crowd, everyone has blind spots.