I can’t remember where I came across this one now, but it’s an interesting pair that I thought got confused in certain circumstances regarding the stirring up of unrest. But wouldn’t you know it – a different picture emerges when I look into it more deeply, and this joins the select ranks of words such as decimated or gunnel that I am not really allowed to be as picky about as I’d like to be!).
To ferment as a verb means to undergo the process of fermentation (“His beer was fermenting nicely and would be ready to drink in a few weeks”) or to stir up disorder. A ferment (as a noun) means social unrest and agitation.
To foment, as a verb, means to instigate or stir up something like strife or revolution – i.e. the same as the secondary meaning of ferment as a verb. What a shame! I think we should still use this for revolutions and ferment for beer, myself (to foment has a nice archaic meaning of bathing a part of the body with a warm or medicated lotion, or fomentation. I doubt any of us will ever use that, but it’s quite jolly, in my opinion).
You can find more troublesome pairs here and the index to them all so far is here.