In the O'Reilly book on Scala, I read that copy-pasting of code creates:
... creates issues of code-bloat, maintainability and skew,
I can't see any obvious definitions of the word skew in context. I am aware of the problems of copy-pasting, but I can't make them fit with the word 'skew'. Since this is used in an O'Reilly book I'm guessing it's a real programming term in circulation.
Skew in its most traditional definition means out of alignment (not parallel). Obviously when code is copied and pasted the two copies are identical and parallel. This is not an invariant. Changes made to one copy is not guaranteed to make it to the other copy (due to carelessness or ignorance). Now there are two versions of code existing in two different places that are skewed (no longer the same). This can lead to weird bugs when behavior seems to work in one place and not in the other. Keep your code DRY (don't repeat yourself).
EDIT/ADDITION: If you have an urge to copy and paste, more than likely what you should do is abstract the code you are copying into a shared superclass (object oriented) or some sort of included utility function (functional).
Ever play the game Telephone as a kid? You know, the one where each person whispers a phrase to the next person in line and at the end, the last person says the phrase out loud and everyone laughs at how different it has become from the original? That's skew. When you copy and paste code, you inevitably make small changes in one or more of the copies. And the more often you copy and paste the same code, the more the changes build up and the code gets skewed away from the original. I've seen code with hundreds of copy and pasted blocks, each varying from the other by just a few characters, it's nearly impossible to maintain. If you find yourself wanting to cut and paste a section of code, you should probably at least consider putting it into its own function.