diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/FolderEventDeliveryPerfTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/FolderEventDeliveryPerfTests.cs
index b101f5de6..b81ac7f0a 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/FolderEventDeliveryPerfTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/FolderEventDeliveryPerfTests.cs
@@ -35,6 +35,9 @@ namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests;
 ///     compiles against the pre-#855 tree for a before/after comparison.</para>
 /// </remarks>
 [Trait("Category", "Perf")]
+// Shares a collection with the other user of OtOpcUaSdkServer.RingEventQueues (a process-wide switch the perf
+// harness flips), so the wiring assertion never observes it switched off (#870 review).
+[Collection(RingEventQueueSwitchCollection.Name)]
 public sealed class FolderEventDeliveryPerfTests
 {
     private const string OptInVariable = "OTOPCUA_EVENT_PERF";
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/RingEventQueueWiringTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/RingEventQueueWiringTests.cs
index f654cc186..2174f71e7 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/RingEventQueueWiringTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/RingEventQueueWiringTests.cs
@@ -9,6 +9,9 @@ namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests;
 ///     item's queue is a <see cref="RingEventMonitoredItemQueue"/>; the SDK's <see cref="EventQueueHandler"/> always asks
 ///     the server's factory), and data-change queues are still the SDK's own.
 /// </summary>
+// Shares a collection with the other user of OtOpcUaSdkServer.RingEventQueues (a process-wide switch the perf
+// harness flips), so the wiring assertion never observes it switched off (#870 review).
+[Collection(RingEventQueueSwitchCollection.Name)]
 public sealed class RingEventQueueWiringTests
 {
     [Fact]
@@ -23,3 +26,11 @@ public sealed class RingEventQueueWiringTests
         factory.CreateDataChangeQueue(false, 2).ShouldBeOfType<DataChangeMonitoredItemQueue>();
     }
 }
+
+/// <summary>Serialises the tests that read or flip <c>OtOpcUaSdkServer.RingEventQueues</c>.</summary>
+[CollectionDefinition(Name)]
+public sealed class RingEventQueueSwitchCollection
+{
+    /// <summary>The collection name.</summary>
+    public const string Name = "RingEventQueueSwitch";
+}
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/RingEventMonitoredItemQueueParityTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/RingEventMonitoredItemQueueParityTests.cs
index b2d264ccf..677b1f573 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/RingEventMonitoredItemQueueParityTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/RingEventMonitoredItemQueueParityTests.cs
@@ -143,6 +143,26 @@ public sealed class RingEventMonitoredItemQueueParityTests
         Both(q => q.IsEventContainedInQueue(Targets[1]), sdk, ring, "absent").ShouldBeFalse();
     }
 
+    /// <summary>Upper boundary of the window: the entry exactly 1000th from newest is still found; 1001st is not.</summary>
+    [Fact]
+    public void Duplicate_check_includes_the_1000th_newest_entry()
+    {
+        var sdk = new EventMonitoredItemQueue(false, 1, Telemetry);
+        var ring = new RingEventMonitoredItemQueue(1);
+        sdk.SetQueueSize(3000, true);
+        ring.SetQueueSize(3000, true);
+        var outside = new Target();
+        var boundary = new Target();
+        void Add(EventFieldList e) { sdk.Enqueue(e); ring.Enqueue(e); }
+        Add(new EventFieldList { Handle = outside });
+        Add(new EventFieldList { Handle = boundary });
+        for (var i = 0; i < 999; i++) Add(new EventFieldList { Handle = Targets[0] });
+
+        // 1001 entries: `boundary` is the 1000th newest (inside), `outside` the 1001st.
+        Both(q => q.IsEventContainedInQueue(boundary), sdk, ring, "boundary").ShouldBeTrue();
+        Both(q => q.IsEventContainedInQueue(outside), sdk, ring, "outside").ShouldBeFalse();
+    }
+
     /// <summary>Random interleavings of every queue operation, at small sizes (wrap-around, grow, shrink, size 0).</summary>
     [Theory]
     [MemberData(nameof(Seeds))]