In July 2008 I started development on LLBLGen Pro v3's new designer. The first thing I realized was that I needed a good, solid, generic framework to base the new designer on, especially because v3 would introduce a new big feature: model-first entity model development. In short, model-first means that the user starts the designer and can build an entity model from scratch (so no meta-data available whatsoever) and create meta-data and mappings from that entity model, or modify an existing or reverse engineered model by adding new elements. So the user will edit, delete, and do other things which aren't based on any meta-data, but based on theory, thought processes and perhaps trial/error. In short: the user will make changes to a live model in memory and will try to undo and redo these changes during the process. Everywhere. Always. So undo/redo has to be present everywhere, and always in every situation. Removing an element, like an entity definition, should remove all its related and depending elements or at least make them update themselves and undo-ing that removal should restore the original state.
(Sorry English speaking visitor, this post is in Dutch, as it's about a Dutch user group meeting)
Update: I've reworded a sentence as it was too vague. Sorry for that.