Aside from what Susan has posted above, if it's a mass-edit with 1 value, you can always try and edit outside of an editing session.
Just select the rows that need to be changed, and go into "selected view" on the data table. From there, you could right click the column that has the data you're changing and choose calculate field. It will tell you not to do it because it's not undoable, but it does get around the restrictions of editing. Again though, it only works if you're replacing 1 value with 1 value. For example, changing all instances of the word "dog" to "cat". Obviously if you have fish, hippos, and massive man-eating GIS monsters, then it gets a bit more complicated.