For example the theme islandora_default:
Is using the preprocess hook 'islandora_preprocess_islandora_default'
This isn't guarantee to run before other preprocess hooks should some other module want to override it.
Instead we should be using 'template_preprocess_theme'
template_preprocess_HOOK(&$variables): Should be implemented by the module that registers the theme hook, to set up default variables.
This ticket is to do a full audit of all the islandora modules and change them to use the correct preprocess hook.