@@ -318,8 +318,10 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 					this.#initializeIntervalIfNeeded();
 				}
 
-				this.emit('active');
+				// The task is started before the event, so that a listener which enqueues more work sees the
+				// concurrency slot already taken. Emitting first let a listener start a second task past the limit.
 				job();
+				this.emit('active');
 
 				taskStarted = true;
 			}
 
@@ -1465,3 +1465,32 @@ test('.onError() - works when called before adding tasks', async () => {
 		{message: 'Early error'},
 	);
 });
+
+test('an active listener cannot exceed the concurrency limit', async t => {
+	// `active` was emitted before `pending` was incremented, so a listener that enqueued work re-entered the
+	// start check while the limit still looked free, and a second task ran alongside the first.
+	const queue = new PQueue({concurrency: 1});
+	let live = 0;
+	let maxLive = 0;
+	let added = false;
+
+	const task = async () => {
+		live++;
+		maxLive = Math.max(maxLive, live);
+		await delay(10);
+		live--;
+	};
+
+	queue.on('active', () => {
+		if (!added) {
+			added = true;
+			queue.add(task);
+		}
+	});
+
+	await queue.add(task);
+	await queue.onIdle();
+
+	assert.strictEqual(maxLive, 1);
+	assert.strictEqual(queue.size, 0);
+});
Original file line number	Diff line number	Diff line change
`@@ -318,8 +318,10 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT`
`318`	`318`	`this.#initializeIntervalIfNeeded();`
`319`	`319`	`}`
`320`	`320`
`321`		`- this.emit('active');`
	`321`	`+ // The task is started before the event, so that a listener which enqueues more work sees the`
	`322`	`+ // concurrency slot already taken. Emitting first let a listener start a second task past the limit.`
`322`	`323`	`job();`
	`324`	`+ this.emit('active');`
`323`	`325`
`324`	`326`	`taskStarted = true;`
`325`	`327`	`}`