We recently installed 10.3 Desktop and find that certain creates or edits of features where vertices are in close proximity are not retained.There seems to be an auto-smoothing routine happening based on the distance between points. In edit vertices, two points of a polygon placed within 5 map units (in this case feet) resolve to a single point between the two -- even when a Save Edit is performed after the first point is set. The proximity of vertices of adjacent features to which these edited points should snap has no effect.
The zoom level has little relevance. Regardless of the zoom level, the "Insert Vertex" option is disabled for any point within 2.2 units (feet) of an existing vertex.
If I try to create a wedge to merge with the existing feature in order to fill a gap, that effort returns this msg: "The feature could not be created, The geometry is invalid." This is apparently because two of the three required vertices are too close together (< 3') to create an acceptable geometry.
This may have something to do with the feature class' XY Tolerance and XY Resolution. You could compare those to a feature class created in a different version. (In ArcCatalog right click on it an check these values under the "Domain, Resolution and Tolerance" tab.) ...just an idea on where to start looking. -Tobias