When might this error appear during the development of a custom Excel Add-in, and what special considerations exist?

When developing add-ins, the order in which modules and references are loaded is crucial. Ensure the add-in's VBA project is properly structured, with dependencies correctly established. This error can appear if modules are not correctly added to the add-in's project or if the add-in tries to access features or classes before they're initialized. Carefully examine the Startup and Close routines for initialization order issues and references to external libraries or components.