There are some things the Spell Check just isn't ever going to catch. Like when the word you wanted to type is mis-spelled, but is still technically a correct word. For example..
when you type "milk" instead of "mild"
Or "her" instead of "here"
Or "fund" rather than "find"
Or "word" when you meant "world"
Your "error" is actually recognized by Spell Check as a legitimate word and will not be corrected. Until minutes, hours, days, or weeks later you look over what you wrote more carefully and realize what happened. Thanks Spell Check, for skipping over these errors and letting the entire world know that sometimes my fingers type faster than my brain can work. This is what I get for allowing technology to do my proof-reading for me.