diff --git a/app/Actions/CreateBucketAction.php b/app/Actions/CreateBucketAction.php
index b3bf2b9..0397bb7 100644
--- a/app/Actions/CreateBucketAction.php
+++ b/app/Actions/CreateBucketAction.php
@@ -22,6 +22,11 @@ public function execute(
         // Validate type + allocation type constraints
         $this->validateTypeConstraints($type, $allocationType);
 
+        // Enforce one overflow bucket per scenario
+        if ($type === BucketTypeEnum::OVERFLOW && $scenario->buckets()->where('type', BucketTypeEnum::OVERFLOW->value)->exists()) {
+            throw new InvalidArgumentException('A scenario can only have one overflow bucket');
+        }
+
         // Validate allocation value based on type
         $this->validateAllocationValue($allocationType, $allocationValue);
 
diff --git a/app/Http/Controllers/BucketController.php b/app/Http/Controllers/BucketController.php
index 3641f86..19c2b45 100644
--- a/app/Http/Controllers/BucketController.php
+++ b/app/Http/Controllers/BucketController.php
@@ -80,6 +80,15 @@ public function update(Request $request, Bucket $bucket): JsonResponse
         $type = BucketTypeEnum::from($validated['type']);
         $allocationType = BucketAllocationTypeEnum::from($validated['allocation_type']);
 
+        // Prevent changing overflow bucket's type away from overflow
+        // (changing TO overflow is handled by validateBucketTypeConstraints below)
+        if ($bucket->type === BucketTypeEnum::OVERFLOW && $type !== BucketTypeEnum::OVERFLOW) {
+            return response()->json([
+                'message' => 'Validation failed.',
+                'errors' => ['type' => ['The overflow bucket\'s type cannot be changed.']],
+            ], 422);
+        }
+
         $constraintError = $this->validateBucketTypeConstraints($type, $allocationType, $bucket->scenario, $bucket);
         if ($constraintError) {
             return $constraintError;
@@ -115,6 +124,12 @@ public function update(Request $request, Bucket $bucket): JsonResponse
      */
     public function destroy(Bucket $bucket): JsonResponse
     {
+        if ($bucket->type === BucketTypeEnum::OVERFLOW) {
+            return response()->json([
+                'message' => 'The overflow bucket cannot be deleted.',
+            ], 422);
+        }
+
         $scenarioId = $bucket->scenario_id;
         $deletedPriority = $bucket->priority;
 
diff --git a/database/factories/BucketFactory.php b/database/factories/BucketFactory.php
index 93821ab..f7dada7 100644
--- a/database/factories/BucketFactory.php
+++ b/database/factories/BucketFactory.php
@@ -15,10 +15,10 @@ class BucketFactory extends Factory
 {
     public function definition(): array
     {
+        // Unlimited excluded — use ->overflow() state modifier
         $allocationType = $this->faker->randomElement([
             BucketAllocationTypeEnum::FIXED_LIMIT,
             BucketAllocationTypeEnum::PERCENTAGE,
-            BucketAllocationTypeEnum::UNLIMITED,
         ]);
 
         return [
diff --git a/tests/Unit/Actions/CreateBucketActionTest.php b/tests/Unit/Actions/CreateBucketActionTest.php
index b1567b1..2c86fa5 100644
--- a/tests/Unit/Actions/CreateBucketActionTest.php
+++ b/tests/Unit/Actions/CreateBucketActionTest.php
@@ -151,6 +151,26 @@ public function test_non_overflow_bucket_cannot_use_unlimited_allocation(): void
         );
     }
 
+    public function test_cannot_create_second_overflow_bucket(): void
+    {
+        $this->action->execute(
+            $this->scenario,
+            'First Overflow',
+            BucketAllocationTypeEnum::UNLIMITED,
+            BucketTypeEnum::OVERFLOW
+        );
+
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('A scenario can only have one overflow bucket');
+
+        $this->action->execute(
+            $this->scenario,
+            'Second Overflow',
+            BucketAllocationTypeEnum::UNLIMITED,
+            BucketTypeEnum::OVERFLOW
+        );
+    }
+
     public function test_want_bucket_cannot_use_unlimited_allocation(): void
     {
         $this->expectException(InvalidArgumentException::class);