diff --git a/docs/IMPLEMENTATION_PLAN.md b/docs/IMPLEMENTATION_PLAN.md
index da9f0e2..725230b 100644
--- a/docs/IMPLEMENTATION_PLAN.md
+++ b/docs/IMPLEMENTATION_PLAN.md
@@ -1322,3 +1322,20 @@ cooldowns, so a deploy can re-alert whatever price is sitting on.
    "warm" means to every consumer of `/api/status`, so it wants its own thought.
 
 Recommended order: 1, then 3, then 2. 4 only if the window still bites after 1.
+
+**Stale bar events across a timeframe switch.** `Cannot update oldest data`
+appeared in the console once ticks were live. Switching timeframe races: the
+server answers `subscribe` with a fresh snapshot from one coroutine while
+another is still draining bar events for the timeframe just left, so a 1m bar
+can land after the 1h snapshot. Applied to the 1h series it is older than every
+point in it, and Lightweight Charts throws rather than ignoring it — taking the
+app down instead of dropping one bar. The race predates the tick feed; Level 1
+made bar events ~15x more frequent, which is what surfaced it.
+
+Guarded at both ends. `app.js` honours the `tf` the event already carries and
+drops anything for a timeframe that is no longer selected. `chart.js` refuses a
+bar older than the series' last point regardless of where it came from — a bar
+behind the last one has nothing to contribute. Verified: 36 rapid timeframe
+switches under a live tick feed produce zero errors, and calling
+`candles.update()` directly with a stale bar still throws while the guarded
+`updateBar()` does not.
diff --git a/static/app.js b/static/app.js
index 5beeeaf..0af95ea 100644
--- a/static/app.js
+++ b/static/app.js
@@ -129,6 +129,12 @@ createApp({
           syncVisibleLevels();
           price.value = message.price;
         } else if (message.type === 'bar') {
+          // Switching timeframe races: the server answers `subscribe` with a
+          // fresh snapshot from one coroutine while another is still draining
+          // bar events for the timeframe just left. A 1m bar applied over a 1h
+          // series is older than everything in it, which the chart rejects
+          // outright. The event carries its timeframe, so honour it.
+          if (message.tf && message.tf !== timeframe.value) return;
           chartApi.updateBar(message.bar);
           price.value = message.bar.c;
           status.value.last_bar_t = message.bar.t;
diff --git a/static/chart.js b/static/chart.js
index 81a1ef7..6a3ecb9 100644
--- a/static/chart.js
+++ b/static/chart.js
@@ -165,6 +165,12 @@ class ConfluenceChart {
   }
 
   updateBar(bar) {
+    // update() throws on anything older than the series' last point, which
+    // takes the whole app down rather than dropping one stale bar. Ticks made
+    // this reachable often enough to matter, so refuse it here as well as at
+    // the source: a bar behind the last one has nothing to contribute.
+    const last = this.bars[this.bars.length - 1];
+    if (last && bar.t < last.t) return;
     this.candles.update(this.toCandle(bar));
     if (this.bars.length && this.bars[this.bars.length - 1].t === bar.t) this.bars[this.bars.length - 1] = bar;
     else this.bars.push(bar);