Tangential but interesting: conventional spell checks, in my experience, tend to turn off for all caps words, or words that start with a capital. Turning it on always can be annoying, so you need something smart, which detects common errors, but doesn't hose you with false positives.

Tangential to this tangent: Google Docs spell check catches wrong use of "flower" vs "flour" based on context. Try this: open a new doc, type:

  Flower

No error. Now add:

  Flower, eggs, sugar

Error on flower. But if you do:

  Flower, sugar, eggs

No error. Milk also works.

ML? Or regex?