I've noticed a recent change (I think) in the way the editor behaves, for me at least.
There seems to be some delay after I hit the enter key before I can move the cursor back to the end of the previous line with the mouse. It's not always consistent, but typically if I hit the enter key, I must click several times at the end of the previous line before my cursor will move there. Alternatively, I can click at the beginning of the following line and use the arrow key to move back. This always seems to work.
To reproduce type a line in the editor. Hit the enter key and try to use the mouse to move the cursor back to the end of the line you just entered.
Update: I don't see this in MacOSX/Safari. Looks like some recent patch to IE may be the culprit.