I’m not a big fan of clunky workarounds, but they do have their place in the world.
This one occurred with a SharePoint 2007 site and IE8. Now I know that by default, this should work fine, but I’m not the first person to run across this issue.
It may happen that if you are running IE8, add a Content Editor Web Part (CEWP) to a page, open the Tool Pane and click on the Rich Text Editor button to edit it, you’ll get a script error. The error will look something like this:
Message: Invalid argument.
The workaround is to go into Compatibility View. Once the page refreshes in that view, things will function as expected. I don’t know why, and it doesn’t fix the problem, but you can get your work done. It’ll satisfy my user until we upgrade to SharePoint 2010.