When it's mayonaise. Yes, it's true they are both an eggy emulsion but when restaurants substitute mayonaise for hollandaise they shouldn't be calling it hollandaise.
Some (Windsor Arms) add 'onion juice' to make it more flavourful.
I'm not a purist because I happen to like the idea of fusion cooking and experimenting but is there a certain level of expectation when you order a straight "Eggs Benedict" and there is no further descriptive like "with added roasted onion added to the hollandaise", etc.
What are your thoughts, Chowhounds?