$key = md5($operation['callback'])

populate_operations() hashes over the function (callback) name, so if a function is re-used for several operations --as is the case with Drupal's built-in operations (they all use node_mass_update())-- then all except the last operation will vanish.

Misc:

- I'm using Drupal-dev, which displays PHP warnings when the code isn't E_ALL compliant, and when using this module I see warnings. Expressions such as `$operation['callback arguments']` probably need to be written `isset($operation['callback arguments']) ? $operation['callback arguments'] : NULL;

- 'private' methods aren't supported, and are probably syntax errors, in PHP 4.