Many times I've had the same thing happen when typing a reply or post on TBN. It has always been that one particular website is open in another tab or window. I know who's website it is. I know that all the ads load before any of the content loads. And I know its a poorly built website. And I know its a poorly maintained website. And its set to update almost continuously. Its a pig.
So, humor us. If it happens again, see if any other websites are open, and if they are, try closing them and seeing if that helps.
Spell check in IE has had problems in the past, too, as mentioned.