At least on OS X, the Notes field accepts text with formatting information which sometimes prevents me from seeing what got pasted in the Notes. For example, if I paste something from a terminal window with white text on a blue background, the pasted text is white in colour in the Notes field (which has a white background). Another example would be text from webpages with a large'ish font, so sometimes its not possible to see the entire text that I just pasted into Notes.
But once the entry is saved, all such formatting information is removed and Notes are converted to plaintext. But I have to do this to ensure the correct text remains in the Notes and there are no extraneous whitepsaces.