I have just upgraded to 2013 from 2012. When dimensioning my viewports in paperspace, I will snap using the linear or aligned dimension and it will appear to snap to the first point, but when I snap on the other end, it will go somewhere way off the viewport or give me a "0" dim.
I've checked my snaps and other settings that I know of.
If you are using associative dims and trying to dimension a 3D model, then see this post:
It appears that this is an "issue" in 2013
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