Duplicating code. If you copy/paste a code section to only change a line after that, you are simply duplicating code and making a bigger mess. In the context of the messy room example above, this would be like introducing another chair with a lower base instead of investing in a new chair that is height-adjustable. Always keep the concept of abstraction in your mind and use it when you can.
I’m not sure that abstractions are always a good thing. Introducing abstractions too early can also lead to brittle, conditional and overly-complex design. A good rule of thumb is to only extract a piece of code once it has been repeated X (e.g. 3) times. :)