Knowing that an element is included in the DOM might not be enough for us to properly interact with it: its state also determines whether our action will be able to go through. Example error message: 
ERROR - UserScript page.waitForSelector: Timeout 30000ms exceeded. 
waiting for selector ".contact-form > .form-control" to be visible
  selector resolved to hidden <element>

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Possible causes

  • Obvious possible cause: the element is set to hidden while it shouldn’t. Something is wrong with the element itself.
  • Not-so-obvious possible cause: a different element is hiding the target element without our knowledge.

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How to avoid confusion

Either walk through the execution in headful mode or take screenshots before and after the instruction that has raised the error - this will help you verify whether the application state actually is the one you expect.
Note that this list neither is nor aims to be complete: additional possible causes most likely exist for this error.