It's tempting to think that only slapdash or minor sites can miss the kinds of errors we find, but in fact they can elude both spell checkers and proofreaders at some of the most prestigious sites in the world as well. Here are two examples:
The President of the United States' blog makes a reference to "both sides of the isle" (instead of "aisle").
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The CDC's website has numerous references to "prostrate cancer" (instead of "prostate cancer").
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Each individual word is spelled correctly, but taken together they are wrong. Think of "piece of mind" (instead of "peace"), which according to Google appears over 300,000 times online, and "sneak peak" (instead of "peek") which appears over 900,000 times. At Knowingly, we have identified over 25,000 of these often misspelled phrases that completely get past spell checkers. Search engines unquestionably notice these errors (just try typing "prostrate cancer" into a search engine and see what it says), and many users notice them as well. They are easy to fix, because it is most often a simple search and replace.
Consider the phrase "medial exam", which appears online over 7,000 times. The intended phrase is clearly "medical exam", but a letter was dropped, and since "medial" is still a word, spell checkers miss this error. We have identified over 60,000 of these sorts of errors, which again can be fixed with a simple search and replace.
While typing, a common error is to press the space bar in the wrong place. Usually, the result is two misspelled words that are easy to spot and fix. We have identified 21,000 cases where the resulting two-word phrase consists of two correctly spelled words that are clearly incorrect. The phrase "doe snot" is clearly supposed to be "does not". Our misplaced spaces rule catches those.
In a similar vein, think of the phrase "letter form Santa", which appears on the web around 15,000 times. We have identified 48,000 common cases of transposed letters.
16,000 rules around capitalization. Is it "quiche Lorraine" or "Quiche Lorraine"?
8,000 rules focus on compound words (replacing "jelly fish" with "jellyfish").
3,000 rules that fix cant, can;t, and so forth.
73,000 errors of miscellaneous sorts which include words that aren't really words ("irregardless", I am looking at you), when to use "a" and "an", incorrect plural forms ("geeses"), punctuation problems (",,"), repeated words, and a whole lot more.