Too much time spent on interfaces makes too many special parts, and too much assembly required.
Too much time focusing on the whole with no thought of re-use makes a hard to maintain solid whole.
Too much time spent writing tests makes code that fails where the tests don't cover
Too much time spent writing untested code makes code that fails where the tests should be.
Too much time focusing on the right way to make something Object-Oriented or Functional makes one forget the user interface, or worse, the user.
Too much time finding patterns means code that isn't gets neglected.
Too much time just coding means ignoring the obvious patterns you repeat.