diff --git a/test/sse-connect-timeout.test.ts b/test/sse-connect-timeout.test.ts
index 0f6d3232..b379c2c8 100644
--- a/test/sse-connect-timeout.test.ts
+++ b/test/sse-connect-timeout.test.ts
@@ -194,6 +194,26 @@ describe('SSE connect timeout', () => {
     expect(app.reconnectAttempts).toBe(1);
   });
 
+  it('a late error from a replaced EventSource leaves the connection that replaced it alone', () => {
+    const app = makeApp();
+    app.connectSSE();
+    const first = FakeEventSource.all[0];
+    app.connectSSE(); // e.g. the user pressed Retry
+    const second = FakeEventSource.all[1];
+    // A browser that still fires onerror on the closed first attempt.
+    first.fail();
+    expect(app.eventSource).toBe(second);
+    expect(second.closed).toBe(false);
+    expect(app.reconnectAttempts).toBe(0);
+    // No retry was scheduled, and the second attempt keeps its own deadline.
+    vi.advanceTimersByTime(TIMEOUT - 1);
+    expect(FakeEventSource.all).toHaveLength(2);
+    expect(second.closed).toBe(false);
+    vi.advanceTimersByTime(1);
+    expect(second.closed).toBe(true);
+    expect(app.reconnectAttempts).toBe(1);
+  });
+
   it('connectSSE() while offline cancels the pending timer (the online event reconnects)', () => {
     const app = makeApp();
     app.connectSSE();
diff --git a/test/tile-grid-sse-filter.test.ts b/test/tile-grid-sse-filter.test.ts
index e444f3ee..b733a768 100644
--- a/test/tile-grid-sse-filter.test.ts
+++ b/test/tile-grid-sse-filter.test.ts
@@ -80,7 +80,11 @@ describe('the SSE filter while tiles own the terminal', () => {
     const app = readFileSync(resolve(import.meta.dirname, '../src/web/public/app.js'), 'utf8');
     const start = app.indexOf('const _sseParams = new URLSearchParams(');
     expect(start).toBeGreaterThan(-1);
-    const block = app.slice(start, app.indexOf('this.eventSource = new EventSource(', start));
+    // Both ends must be found: a missing end marker makes slice() take the
+    // rest of app.js, and the checks below would then pass on any text.
+    const end = app.indexOf('const eventSource = new EventSource(', start);
+    expect(end).toBeGreaterThan(start);
+    const block = app.slice(start, end);
     expect(block).toContain('this._sseFilterSessionId(this.activeSessionId)');
     expect(block).not.toMatch(/set\('sessions', this\.activeSessionId\)/);
   });