I find that there are problems with pasting plain ASCII or unicode text into a CKEditor document. By rights, pasting plain text should be functionally equivalent to typing that text manually. What in fact happens is that pasting a block of text into a para or div creates a 'styling island' in which the styling which should be inherited from the parent HTML element is missing from the pasted text.
Easily shown in the online demo: Select a para and apply italic. Now paste text from Notepad. Italic is missing from the pasted text. Obviously this simplistic example is easy to fix. If the paragraph has more complex styling then it is not possible to repair it, except by editing the source.