In the Hydrator class, some exceptions can be thrown with text along the lines of "Couldn't hydrate. Found a non-existent key."

It would be more useful to the end user to make these more verbose, for example by changing

  throw new Doctrine_Hydrator_Exception("Couldn't hydrate. Found a non-existent key.");

to

  throw new Doctrine_Hydrator_Exception("Couldn't hydrate. Found a non-existent key called '$field'.");

There are at least four instances in this file where I think this would help (lines 144, 146, 210 and 212). However, please sanity-check that this makes sense since you know the Doctrine code base far better than I!

C