diff --git a/crates/render/src/cache.rs b/crates/render/src/cache.rs
index 3fabe473..17794d46 100644
--- a/crates/render/src/cache.rs
+++ b/crates/render/src/cache.rs
@@ -521,6 +521,27 @@ fn still_footage(ctx: &EvalCtx, item: effectcraft_project::ItemId) -> bool {
     ctx.project.item(item).is_some_and(|it| it.proxy.is_none() && matches!(&it.kind, ItemKind::Footage(f) if f.kind == FootageKind::Still))
 }
 
+/// Does the layer have an animated mask that motion blur may draw at other times (Layer ▸ Mask ▸
+/// Motion Blur On, or Same As Layer with the layer blurred)?
+fn mask_motion_blur(ctx: &EvalCtx, layer: &Layer, blur: bool) -> bool {
+    use effectcraft_project::{GroupKind, MaskMotionBlur};
+    fn animated(g: &PropGroup) -> bool {
+        g.children.iter().any(|c| match c {
+            Node::Prop(p) => !p.keys.is_empty() || p.has_expression(),
+            Node::Group(sub) => animated(sub),
+        })
+    }
+    ctx.comp.enable_motion_blur
+        && layer.masks().is_some_and(|m| {
+            m.groups().any(|g| match g.kind {
+                GroupKind::Mask { motion_blur, .. } => {
+                    (motion_blur == MaskMotionBlur::On || (motion_blur == MaskMotionBlur::SameAsLayer && blur)) && animated(g)
+                }
+                _ => false,
+            })
+        })
+}
+
 fn has_expression(g: &PropGroup) -> bool {
     g.children.iter().any(|c| match c {
         Node::Prop(p) => p.has_expression(),
@@ -635,6 +656,11 @@ fn key_any(ctx: &EvalCtx, layer: &Layer, scale: f64, draft: bool, blur: bool, fo
             effectcraft_effects::paint::cache_key(g, lt).0.hash(&mut h);
         }
     }
+    // Mask motion blur draws animated masks at the shutter's other times too: their values at
+    // this time don't tell two frames apart (a hold before a move).
+    if mask_motion_blur(ctx, layer, blur) {
+        layer.layer_time(ctx.time).0.hash(&mut h);
+    }
     if time_dependent(layer, effects != Some(0)) {
         layer.layer_time(ctx.time).0.hash(&mut h);
         // Effects also see the layer's time mapping.
diff --git a/crates/render/src/tests_mask_blur.rs b/crates/render/src/tests_mask_blur.rs
index 12b88bbb..07bc1e95 100644
--- a/crates/render/src/tests_mask_blur.rs
+++ b/crates/render/src/tests_mask_blur.rs
@@ -41,6 +41,31 @@ fn alpha_at_25(p: &Project, cid: ItemId, motion_blur: bool) -> f32 {
     Renderer::new(p, &NoFootage, RenderOpts { motion_blur, ..Default::default() }).comp_frame(cid, half).get(25, 50)[3]
 }
 
+/// A mask that holds from frame 0 to 2, then moves: frames 1 and 2 have the same mask value, but at
+/// frame 2 the shutter already sees it move. The layer cache must not serve frame 1's layer there.
+#[test]
+fn mask_motion_blur_keys_the_layer_cache_by_time() {
+    let (mut p, cid) = project(true);
+    let rect = |x: f64| Value::Path(ShapePath::rect([x, 50.0], 20.0, 100.0));
+    let l = &mut p.comp_mut(cid).unwrap().layers[0];
+    let Some(Node::Group(m)) = l.props.sub_mut("masks").unwrap().children.first_mut() else { panic!("mask") };
+    m.get_mut("path").unwrap().keys = vec![
+        Keyframe::new(Tick::ZERO, rect(20.0)),
+        Keyframe::new(FrameRate::FPS_30.tick_of(2), rect(20.0)),
+        Keyframe::new(FrameRate::FPS_30.tick_of(3), rect(60.0)),
+    ];
+    let cache = crate::LayerCache::default();
+    let render = |t: Tick, cached: bool| {
+        let mut r = Renderer::new(&p, &NoFootage, RenderOpts { motion_blur: true, ..Default::default() });
+        r.cache = cached.then_some(&cache);
+        r.comp_frame(cid, t)
+    };
+    let (f1, f2) = (FrameRate::FPS_30.tick_of(1), FrameRate::FPS_30.tick_of(2));
+    assert_ne!(render(f1, false), render(f2, false), "the shutter at frame 2 sees the move");
+    render(f1, true);
+    assert_eq!(render(f2, true), render(f2, false));
+}
+
 #[test]
 fn mask_motion_blur_follows_the_render_and_the_comp() {
     let (p, cid) = project(true);