I'm working on a different drawing now and was running into a similar problem. See the attached screenshot. I have my polar angle snaps set to 45 and 90. Snapang is 0.00 and, as you can see, even though the dotted line *looks* vertical, the actual angle is 89.97 instead of 90, which messes up everything else I would reference off of that line. Of course, I can't re-create that error now that I'm trying to do it, but who knows how often that happens without my noticing?