What hurts maintainability? [duplicate]

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• How would you know if you've written readable and easily maintainable code? 19 answers

For someone who doesn't have much real world experience yet, the notion of maintainable code is a bit vague, even though it follows from typical good practice rules. Intuitively I can tell that code can be well written, but not maintainable if for example it hard-wires information that is subject to constant change, but I still have a hard time looking at code and deciding if it's maintainable or not.

So the question is: what hurts maintainability? What should I look for?