Robert Somppi provides a tip to help 2D AutoCAD users ensure their z values stay at zero.
"If you have ever inadvertently snapped to a point that has a non-zero z value when you were drawing in 2D, here's one for you. Non-planar lines look like their linetype scales have been reduced. If you draw primarily in 2D and work with files from others that are not in 2D (either by design or bad CAD-ing), you can prevent AutoCAD from snapping to a non-zero z coordinate — sort of. Actually, using this option tells AutoCAD to replace that z value with the current elevation; if your drawing is in 2D, then that elevation should be zero.
"In Options, go to the Drafting tab. In the bottom left is Object Snap Options. Select Replace Z value with current elevation. This works if you draw in 3D but may not be desirable, depending on how you work."
Notes from Cadalyst tip reviewer Brian Benton: This is one of those settings that users forget is there. If you only draw in two dimensions, you probably want to turn this on. If you draw in 2D most of the time, you should consider turning this on. If you work in 3D and want to draw in the same plane while drawing, turn this on. If you need to draw in three dimensions, turning it on may cause more headaches than it will prevent. Regardless of how you draw, this tip could really help you out.