I had the same problem. I think one of the moderators was kind enough to tell us it had something to do with IE. if it happens again click on the button on the top very far right. The one with the big blue A, and the little black A. it fixed the problem for me. it will still do it from time to time, but clicking on the A's always helps.
I made the mistake of going to IE10, and it seems to have caused some issues. One that stands out for me is I cannot get the cursor to move to have spaces, like a new paragraph. If I am quoting some one I cannot type before the quote, so my words are after the quote. Little bit leery of going to IE11 now. Also cannot remove IE10.