diff --git a/extractrs-core/src/bbox.rs b/extractrs-core/src/bbox.rs
index ad8c507..d65ea9c 100644
--- a/extractrs-core/src/bbox.rs
+++ b/extractrs-core/src/bbox.rs
@@ -154,6 +154,16 @@ impl BBox {
     /// Uses quadrant-based projection with clamping for numerical robustness.
     ///
     /// Port of exactextract's `Box::crossing()` (box.cpp:43-118).
+    ///
+    /// **Degenerate fallback:** the original exactextract implementation
+    /// throws "Never get here" when a vertical/horizontal segment lies
+    /// entirely outside the box (i.e., the line never intersects this box).
+    /// This can be triggered by low-quality polygon data with collinear
+    /// vertices that happen to fall outside the current cell — see
+    /// isciences/exactextract#126. Rather than panic, we return the box
+    /// side closest to the segment so the traversal can make progress.
+    /// Callers should also avoid invoking `crossing()` in this state when
+    /// possible; see `Cell::take()` for the upstream guard.
     pub fn crossing(&self, c1: &Coord, c2: &Coord) -> Crossing {
         // Vertical line (x constant)
         if c1.x == c2.x {
@@ -162,7 +172,14 @@ impl BBox {
             } else if c2.y <= self.ymin {
                 return Crossing::new(Side::Bottom, c1.x, self.ymin);
             } else {
-                unreachable!("vertical line doesn't exit box");
+                // Degenerate: vertical line entirely outside the box on x.
+                // Return the closest side so the traversal can advance.
+                let y = clamp(c2.y, self.ymin, self.ymax);
+                if c1.x <= self.xmin {
+                    return Crossing::new(Side::Left, self.xmin, y);
+                } else {
+                    return Crossing::new(Side::Right, self.xmax, y);
+                }
             }
         }
 
@@ -173,7 +190,13 @@ impl BBox {
             } else if c2.x <= self.xmin {
                 return Crossing::new(Side::Left, self.xmin, c1.y);
             } else {
-                unreachable!("horizontal line doesn't exit box");
+                // Degenerate: horizontal line entirely outside the box on y.
+                let x = clamp(c2.x, self.xmin, self.xmax);
+                if c1.y <= self.ymin {
+                    return Crossing::new(Side::Bottom, x, self.ymin);
+                } else {
+                    return Crossing::new(Side::Top, x, self.ymax);
+                }
             }
         }
 
@@ -290,6 +313,40 @@ mod tests {
         assert!((c.coord.y - 1.0).abs() < 1e-10);
     }
 
+    #[test]
+    fn test_crossing_vertical_degenerate_left_of_box() {
+        // Vertical segment whose x is strictly left of the box.
+        // Pre-fix this would panic with "vertical line doesn't exit box".
+        let b = unit_box();
+        let c = b.crossing(&Coord::new(-2.0, 0.5), &Coord::new(-2.0, 0.5));
+        assert_eq!(c.side, Side::Left);
+        assert_eq!(c.coord.x, 0.0);
+    }
+
+    #[test]
+    fn test_crossing_vertical_degenerate_right_of_box() {
+        let b = unit_box();
+        let c = b.crossing(&Coord::new(5.0, 0.5), &Coord::new(5.0, 0.5));
+        assert_eq!(c.side, Side::Right);
+        assert_eq!(c.coord.x, 1.0);
+    }
+
+    #[test]
+    fn test_crossing_horizontal_degenerate_above_box() {
+        let b = unit_box();
+        let c = b.crossing(&Coord::new(0.5, 5.0), &Coord::new(0.5, 5.0));
+        assert_eq!(c.side, Side::Top);
+        assert_eq!(c.coord.y, 1.0);
+    }
+
+    #[test]
+    fn test_crossing_horizontal_degenerate_below_box() {
+        let b = unit_box();
+        let c = b.crossing(&Coord::new(0.5, -2.0), &Coord::new(0.5, -2.0));
+        assert_eq!(c.side, Side::Bottom);
+        assert_eq!(c.coord.y, 0.0);
+    }
+
     #[test]
     fn test_crossing_diagonal_steep_up_right() {
         let b = unit_box();
diff --git a/extractrs-core/src/cell.rs b/extractrs-core/src/cell.rs
index 44518eb..214845b 100644
--- a/extractrs-core/src/cell.rs
+++ b/extractrs-core/src/cell.rs
@@ -92,7 +92,19 @@ impl Cell {
         // Coordinate exits cell.
         // Use prev_original for crossing computation to avoid robustness issues
         // from interpolated coordinates (exactextract cell.cpp:126-129).
-        let crossing = if let Some(prev) = prev_original {
+        //
+        // Skip prev_original if it would form a degenerate vertical/horizontal
+        // segment lying entirely outside the cell — last_coordinate is always
+        // on the boundary or interior so it's a safe fallback. See
+        // isciences/exactextract#126 for the data conditions that trigger this.
+        let prev_safe = prev_original.filter(|prev| {
+            let vertical_outside = prev.x == c.x
+                && (prev.x < self.bbox.xmin || prev.x > self.bbox.xmax);
+            let horizontal_outside = prev.y == c.y
+                && (prev.y < self.bbox.ymin || prev.y > self.bbox.ymax);
+            !vertical_outside && !horizontal_outside
+        });
+        let crossing = if let Some(prev) = prev_safe {
             self.bbox.crossing(prev, c)
         } else {
             let last = *self.traversals[idx].last_coordinate();