I am also facing the same problem.i have checked all the coordinates are same.i tried restarting the progrma bt it is still not working..i am not being able to make a single feature using any feature class.
I was having this same problem. Found out the problem was... my next line was only 50+ feet and I had SNAPPING turned on. When entered, the line immediately snapped back to ZERO length -invalid geometry. Turned SNAPPING OFF - problem fixed.