diff --git a/crates/render/src/geometry.rs b/crates/render/src/geometry.rs
--- a/crates/render/src/geometry.rs
+++ b/crates/render/src/geometry.rs
@@ -199,3 +199,21 @@ pub(crate) fn scale_of(m: Affine) -> f64 {
 pub(crate) fn blur(sigma: f64) -> Filter {
     Filter::from_primitive(FilterPrimitive::GaussianBlur { std_deviation: sigma.clamp(0.0, 500.0) as f32, edge_mode: EdgeMode::None })
 }
+
+/// The widest a shape may reach on screen, in pixels, and still be drawn: ten thousand screens
+/// across, far past any real deck, and still cheap for vello to flatten (#1165).
+const MAX_DEVICE_EXTENT: f64 = 1e7;
+
+/// Whether a shape's geometry can go to vello. Geometry far beyond any screen (a size at the EMU
+/// limit, group scales compounding, custom-geometry guides) makes vello's flattener size a buffer
+/// by its on-screen length, and the allocation aborts the process (#1165). Each path is checked
+/// for finite points first: a bounding box drops NaN, and a union with the frame would hide it.
+/// The extent is the control box, which always holds the curve: the tight bounding box can miss
+/// a curve's extremum when finding it overflows.
+pub(crate) fn drawable(geo: &Geometry, m: Affine, w: f64, h: f64) -> bool {
+    if !geo.paths.iter().all(|p| p.path.is_finite()) {
+        return false;
+    }
+    let dev = m.transform_rect_bbox(geo.outline().control_box().union(Rect::new(0.0, 0.0, w, h)));
+    dev.is_finite() && dev.width().max(dev.height()) <= MAX_DEVICE_EXTENT
+}
diff --git a/crates/render/src/renderer.rs b/crates/render/src/renderer.rs
--- a/crates/render/src/renderer.rs
+++ b/crates/render/src/renderer.rs
@@ -1,4 +1,6 @@
-use super::geometry::{Caster, blur, border_outside, camera_turn, color, fill_opacity, outside_line, scale_of, shadow_local, shape_geometry};
+use super::geometry::{
+    Caster, blur, border_outside, camera_turn, color, drawable, fill_opacity, outside_line, scale_of, shadow_local, shape_geometry,
+};
 use super::opts::{RenderOpts, ShapeState, SlideFields};
 use super::raster::Image;
 use super::slide::{SlidePart, needs_filters, output_size};
@@ -542,6 +544,10 @@ impl Renderer {
             None => (self.scaled_geometry(s, m, depth, w, h), Vec::new()),
         };
         let fade = |i: usize| fades.get(i).copied().unwrap_or(deckcraft_geom::morph::Fade { fill: 1.0, stroke: 1.0 });
+        if !drawable(&geo, m, w, h) {
+            log::warn!("render: shape {} skipped: its outline is not finite or far larger than any screen", s.id.0);
+            return;
+        }
         let (fill, fill_ph) = resolve::fill(rctx, s);
         let (line, line_ph) = resolve::line(rctx, s);
         let (effects, fx_ph) = resolve::effects(rctx, s);
diff --git a/crates/render/src/tests.rs b/crates/render/src/tests.rs
--- a/crates/render/src/tests.rs
+++ b/crates/render/src/tests.rs
@@ -2951,3 +2951,59 @@ fn pie_data_labels_take_the_chart_text_colour_when_it_has_one() {
     assert!(count(&coloured, &red) > 30, "the labels take the chart's red: {} red pixels", count(&coloured, &red));
     assert!(count(&plain, &white) > count(&coloured, &white), "the labels were white before");
 }
+
+/// #1165: an ellipse sized at the EMU limit, crossing the slide, made vello's flattener allocate a
+/// buffer from its on-screen length and abort. It is skipped; the rest of the slide still draws.
+#[test]
+fn a_shape_far_larger_than_any_screen_is_skipped_not_flattened() {
+    let huge = Shape {
+        id: ShapeId(501),
+        geom: Geom::preset("ellipse"),
+        xfrm: Some(Xfrm::new(-5e17, -5e17, 1e18, 1e18)),
+        style: Some(ShapeStyle::accent(deckcraft_color::SchemeSlot::Accent2)),
+        ..Default::default()
+    };
+    let normal = Shape {
+        id: ShapeId(502),
+        xfrm: Some(Xfrm::new(100.0, 100.0, 200.0, 100.0)),
+        style: Some(ShapeStyle::accent(deckcraft_color::SchemeSlot::Accent1)),
+        ..Default::default()
+    };
+    let p = deck_with(vec![huge, normal]);
+    let img = render_slide(&p, 0, &RenderOpts { scale: 1.0, ..Default::default() });
+    let accent = p.masters[0].theme.colors.get(deckcraft_color::SchemeSlot::Accent1);
+    let px = img.pixel(200, 150);
+    assert!((px[0] as i32 - accent.r as i32).abs() <= 2, "{px:?} vs {accent:?}");
+    assert_eq!(img.pixel(50, 50), [255, 255, 255, 255], "the huge ellipse is not drawn");
+}
+
+/// #1165: a path with a NaN point is not drawn, even when the shape's frame is finite and small
+/// (a bounding box, or a union with the frame, would hide the NaN).
+#[test]
+fn a_shape_whose_outline_is_not_finite_is_not_drawable() {
+    let geo = |pts: &[(f64, f64)]| {
+        let mut path = kurbo::BezPath::new();
+        path.move_to(pts[0]);
+        for p in &pts[1..] {
+            path.line_to(*p);
+        }
+        deckcraft_geom::preset::Geometry {
+            paths: vec![deckcraft_geom::preset::SubPath { path, fill: deckcraft_geom::preset::FillMode::Norm, stroke: true, even_odd: false }],
+            text_rect: kurbo::Rect::ZERO,
+            handles: vec![],
+            sites: vec![],
+        }
+    };
+    let m = kurbo::Affine::IDENTITY;
+    assert!(crate::geometry::drawable(&geo(&[(0.0, 0.0), (100.0, 50.0)]), m, 100.0, 50.0));
+    assert!(!crate::geometry::drawable(&geo(&[(0.0, 0.0), (f64::NAN, 50.0)]), m, 100.0, 50.0));
+    assert!(!crate::geometry::drawable(&geo(&[(0.0, 0.0), (f64::INFINITY, 50.0)]), m, 100.0, 50.0));
+    assert!(!crate::geometry::drawable(&geo(&[(0.0, 0.0), (1e18, 50.0)]), m, 100.0, 50.0));
+    // A quadratic whose extremum overflows kurbo's bounding box: its control point still counts.
+    let mut path = kurbo::BezPath::new();
+    path.move_to((0.0, 0.0));
+    path.quad_to((1e308, 0.0), (0.0, 0.0));
+    let mut quad = geo(&[(0.0, 0.0), (1.0, 1.0)]);
+    quad.paths[0].path = path;
+    assert!(!crate::geometry::drawable(&quad, m, 100.0, 50.0));
+}