I've had similar observations and talked with ESRI Support about it. Essentially what is going on is that at a scale of less than one to one the software can no longer visually display the linework correctly. So what is being viewed is not really an accurate depiction.
Of course, seeing these apparent slivers (and gaps) can be a bit disconcerting. Visual review is one way people see issues, so the question then becomes, is the issue real or not?
So here is one way to check. Measure the gap with the Measure Tool. As mentioned by Micah Babinski the XY Tolerance controls how close linework is to be considered coincident. If the distance measured turns out to be less than the XY Tolerance, then the linework is coincident, even though way zoomed in it looks like there is a problem.
Also, more generally, if one sees issues, check the scale right off the bat. If it is less than 1:1, realize that what is being displayed may not be an accurate depiction.
For more specifics, check out the reply from ESRI Support when I posed similar issues I noticed to them:
As per our phone interaction, a map scale past 1:1 is a known limitation of our software because at a scale of 1:1, the screen display is equal to the actual distance of objects on the earth's surface (i.e. an inch on your screen is equal to an inch on the ground). When you go past that scale, it may appear that features are overlapping (when in reality) what you are seeing is the actual software resolution tolerance pushed past it's maximum threshold. In essence, the entire screen is only one point (at a map scale of 1:0), which is why your work flow is producing inaccurate results.
Strangeness - gaps unintentionally created while editing
Chris Donohue, GISP