diff --git a/apps/game/src/gameplay/galaxy/ui.rs b/apps/game/src/gameplay/galaxy/ui.rs
index 5838394..e1db94a 100644
--- a/apps/game/src/gameplay/galaxy/ui.rs
+++ b/apps/game/src/gameplay/galaxy/ui.rs
@@ -735,7 +735,7 @@ pub fn param_button_handler(
 /// Helper: get a mutable reference to the currently-selected disk.
 /// Clamps the index to the valid range as a safety measure.
 fn selected_disk_mut(params: &mut GalaxyParams, index: usize) -> &mut DiskParams {
-    let idx = index.min(params.disks.len() - 1);
+    let idx = index.min(params.disks.len().saturating_sub(1));
     &mut params.disks[idx]
 }
 
@@ -770,8 +770,9 @@ pub fn refresh_control_panel_values(
     mut values: Query<(&ParamValue, &mut Text)>,
 ) {
     // Resolve the selected disk, clamped to a valid index.
-    let disk_idx = selected_disk.0.min(params.disks.len() - 1);
-    let disk = &params.disks[disk_idx];
+    let Some(disk) = params.disks.get(selected_disk.0.min(params.disks.len().saturating_sub(1))) else {
+        return;
+    };
 
     for (marker, mut text) in &mut values {
         let new = match marker.0.as_str() {
@@ -931,7 +932,7 @@ fn spawn_scroll_contents_with_tabs(
     spawn_section(scroll, "Disk");
 
     // Tab bar: [1] [2] ... [+]
-    let tab_bar_active = active_disk.min(params.disks.len() - 1);
+    let tab_bar_active = active_disk.min(params.disks.len().saturating_sub(1));
     scroll
         .spawn(Node {
             width: Val::Percent(100.0),
@@ -1270,7 +1271,7 @@ fn randomize_disk(rng: &mut impl rand::Rng) -> DiskParams {
     // show distinct spiral structure. The floor ensures at least ~0.6 turns
     // between adjacent arms.
     let twist_floor = disk.arms as f32 * 4.0;
-    let effective_min = DISK_TWIST_MIN.max(twist_floor);
+    let effective_min = DISK_TWIST_MIN.max(twist_floor).min(DISK_TWIST_MAX);
     disk.twist = rng.gen_range(effective_min..=DISK_TWIST_MAX);
     disk.twist = (disk.twist / DISK_TWIST_STEP).round() * DISK_TWIST_STEP;