Can someone answer this question. It is not peculiar to any version of Arcmap desktop: I have seen it in 10.0, 10.1,10.2,10.3 and 10.4.
When I "start editing" from the editor toolbar I get a dialogue box asking which layer or workspace I wish to edit.
After I choose one I get another warning dialogue box which lists all (well, all, as far as I can tell) the layers in the frame and where each one has a message "Spatial reference does not match data frame". There are many layers in my frames because I am using Business Analyst basemaps which have multiple datasets which display at different scale values.
Why do I get this message? Both the data frame and the layers are using the same geographic coordinate system (WGS_84), although the former is projecting the data using the World Mercator projection.
The warnings don't prevent my being able to edit the data: I am only editing fields which are not geographic. But the message seems to imply that if I were digitizing new features that I would be met with failure.