I run into this question usually on a weekly basis so I thought it would be a good post.

When creating a legend, say a door frame legend, and there needs to be a reference a detail in that legend Revit wont allow it.  The section and callout commands are greyed out.  Don’t use a dummy bubble (never ever use a dummy bubble) instead go to another live view of the model (i.e. section, plan, elevation), choose the correct command section or callout, and do a reference view.

Copy that view reference to the clipboard, switch to the legend view and paste from the clipboard.