I have run into this before and simply marked it as an exception and moved on, but it puzzles and bothers me a bit so if someone can provide a reasonable explanation, that would help me. Surprisingly, I couldn't find a post related to it here or in Esri's online documentation.
When I create a topology and add the rule "Must Not Have Gaps", even with this 2-feature polygon feature class, there seems to always be one error that encompasses the entire feature class.
Can this error be effectively fixed?
When I have it create a feature (a common fix for topology issues), it creates a single polygon that includes the area of both features but not the boundary between them and the error does not appear to include any part of the boundary between the features as you can see in the screenshot.
I remember in the days of "Coverages" and ArcInfo Workstation that there was this concept of a "World Polygon" that represented the universe outside of your extent (and you were NOT to delete it). The only "reason" (for lack of a better word) for what I am seeing is that this is a manifestation or result of that same concept. Maybe it should be marked as an exception and I should just move on as I've done in the past. Comments suggesting things to try (especially if this isn't exactly the "non-error" that I've described), are welcome. I'm using ArcMap 10.6 for this.