If you go to http://jsfiddle.net/r9HCt/ (which displays the ck editor demo page in an iframe), and hit enter inside of the editor, it will result in two line breaks and a space. The line breaks will be before the cursor, and the space will be after it.
Browsers other than Firefox don't seem to have the same problem (they only insert one line break). The demo page alone outside of the iframe works correctly on Firefox.
I was able to reproduce this with demo page and nightly version. I have tried local samples and didn't get this problem so it seems to only work with local editor. Problem can be reproduced from CKEditor 4.0.
Two paragraphs are inserted after cursor and BR is inserted before it.
I had the same problem as discribed, CKE on iframe (in my case it's a canvas App iframe on Facebook) in Mozilla inserts extra paragraph's and even extra <li> when using <ol> or <ul> tag. Is someone has an idea how to fix it?