Why did my profile cut on the wrong side of the line, or cut both sides of the same line?

Why this happens

1. Open vectors don't have an inside or outside. For open vectors the profile toolpath is cut to the Left, to the Right, or On the vector (official documentation), and Left and Right depend on the start node and direction of the vector. Flip the direction and the cut side flips.
2. A duplicate vector is stacked on top. Two identical vectors means the toolpath machines both, giving a cut on each side of the "one" line, or a slot twice the expected width.
3. A vector that loops back over itself creates ambiguous sides.

The fix

1. Close your vectors whenever possible. Closed shapes make Inside and Outside unambiguous.
2. For deliberately open vectors: enter Node Edit (N), hover over the end you want as the start, and press P to set the start point there. That flips the machining side. (This keyboard method comes from the software maker's user forum; check it in your version.)
3. Find stacked duplicates: select the vector. A single selected vector shows dotted or dashed; if it still looks solid, there is a twin underneath. Delete it. The Vector Validator also flags duplicates.

Pro tips

• Parts undersized by exactly one bit diameter usually mean the cut was on the wrong side; undersized by one bit radius usually means it cut "On" the line instead of Outside.
• Imported and traced files are the usual source of stacked duplicates. See the duplicate-vectors entry.