An issue that is sometimes found in AutoCAD 2D drawings is the dreaded Z values.
This is because normal AutoCAD is mostly used in 2D,
This means that only the XY plain is being used and any Z value that creeps in,
Could cause us problems.
The image below looks ok until you change the view to a side view,
You do this by selecting one of the faces on the view cube or cardinal points around it.
Now we can see that the drawing that looked nice and flat has several different Z heights.
If we where to draw lines between certain points, we may end up with lines drastically longer than they appear from the top view.
A quick option to fix this mess,
Is to switch back to the Top view, looking onto the XY Plane,
This is critical that you are looking directly down on the XY Plane,
If you are looking at it at a different angle,
The view will be projected skew.
Use the FLATTEN command.
Select all the lines/objects that you need to flatten,
When you are done with the selection and hit enter,
The prompt for Remove hidden lines will pop up.
As you may guess this will try to remove hidden lines and objects though this only really works in certain situations.
Type Y or N and hit enter.
The objects should now be flattened,
To check this, flip your view to the side by selecting a face on the view cube or orbiting around.