Drupal: handler is broken or missing

While developing a Drupal 7 module at work, I got the following error message:

The handler for this item is broken or missing and cannot be used. If a module provided the handler and was disabled, re-enabling the module may restore it. Otherwise, you should probably delete this item.

The actual problem was that the hook method I used was declared in a file that was not listed in the module's .info file. After adding it there the error was gone:

files[] = mymodule_handler_file.inc

Written by Christian Weiske.

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