They'll be some confusion between Core Rules, Modules, and House Rules though. Some people are just ridiculously offended and defensive when you call anything in their personal home-brew scenarios "house rules." They've kind of latched onto an "everything is optional!" mantra to reject this horribly traumatic label.
There's Core Rules - the underlying premises on which you build your custom D&D game as opposed to something like Vampire or GURPS. Modules are built to fit onto these premises - adding to or subtracting from them with a deliberate purpose and professional development and balancing.
House Rules are things Players and DM's implement outside of Core and Modules in order to make their game more fun. Maybe they want to play historic Greyhawk so they don't have Dragonborn. Maybe they hate the idea of "damage on a miss" or "unlimited magic" so they start banning the Slayer feat and all Cantrip spells. These kind of a la carte changes to the Core Rules without the benefit of professional development, balance, or publishing are precises House Rules and they come with no warranties.
If enough tables have to adapt House Rules to address a particular game feature that just doesn't work, then you probably have a design defect.
"This PHB comes with Dragonborn. I hate Dragonborn. People shouldn't call it a House Rules when I ban Dragonborn," is not a design defect. (Substitute whatever pet-peeve you want for the word "Dragonborn" - "automatic damage," "hit dice," "clerics with shields," "warlords," whatever.)
- Marty Lund
