A spell checker does not check the word in context. It will pass both whether and weather, too or two. I had seen this a long time ago and went googling for it.
Here is The Ode To The Spell-Checker
From a very interesting looking Leslie's Omnibus-- Blithely rolling off the email trail and onto the information highway.
From Elizabeth -- Ode To The Spell-Checker
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Eye halve a spelling chequer
It came with my pea sea
It plainly marques four my revue
Miss steaks eye kin knot sea.
Eye strike a key and type a word
And weight four it two say
Weather eye am wrong oar write
It shows me strait a weigh.
As soon as a mist ache is maid
It nose bee fore two long
And eye can put the error rite
Its rare lea ever wrong.
Eye have run this poem threw it
I am shore your pleased two no
Its letter perfect awl the weigh
My chequer tolled me sew.
_____
Heh...