This happens to me a lot as well! I can't seem to figure out exactly what causes it, but sometimes I will click a line endpoint with end snaps turned on and the coordinate it snaps to is somewhere close to zero, usually like 23,15 or something. If I use a right click snap and choose end point and then click the end of the line it selects the right coordinate. I agree its very annoying. THANKFULLY the incorrect coordinate is way off and very apparent, this always makes me extremely nervous that at some point in time maybe the same error existed but only slightly off.