FCKEditor in DotNetNuke - Javascript error in Internet Explo

I'm no real programmer, but I did find that when using the editor to create a letter I was getting that error too. After much trial and error I found that I could not have one line that was in Bold, underlined and red in Large text when the surrounding text was in medium. If I changed it to medium it worked. It may have been caused from a number of other possibilities that I am not connecting or have time to experiment with, but I'm sure someone more versed in this may know what was going on. I just don't know if it was a bug or not with the editor ver. 2.4.2 build 14978.