diff --git a/CHANGES/2567.bugfix b/CHANGES/2567.bugfix
new file mode 100644
index 00000000000..b6c235d87e9
--- /dev/null
+++ b/CHANGES/2567.bugfix
@@ -0,0 +1 @@
+Return client connection back to free pool on error in `connector.connect()`.
diff --git a/aiohttp/connector.py b/aiohttp/connector.py
index c67ad78bf92..8fbc0a152d0 100644
--- a/aiohttp/connector.py
+++ b/aiohttp/connector.py
@@ -283,6 +283,15 @@ def _cleanup(self):
             self._cleanup_handle = helpers.weakref_handle(
                 self, '_cleanup', timeout, self._loop)
 
+    def _drop_acquired_per_host(self, key, val):
+        acquired_per_host = self._acquired_per_host
+        if key not in acquired_per_host:
+            return
+        conns = acquired_per_host[key]
+        conns.remove(val)
+        if not conns:
+            del self._acquired_per_host[key]
+
     def _cleanup_closed(self):
         """Double confirmation for transport close.
         Some broken ssl servers may leave socket open without proper close.
@@ -354,7 +363,7 @@ def connect(self, req):
 
         if self._limit:
             # total calc available connections
-            available = self._limit - len(self._waiters) - len(self._acquired)
+            available = self._limit - len(self._acquired)
 
             # check limit per host
             if (self._limit_per_host and available > 0 and
@@ -396,15 +405,16 @@ def connect(self, req):
                     raise ClientConnectionError("Connector is closed.")
             except:
                 # signal to waiter
-                for waiter in self._waiters[key]:
-                    if not waiter.done():
-                        waiter.set_result(None)
-                        break
+                if key in self._waiters:
+                    for waiter in self._waiters[key]:
+                        if not waiter.done():
+                            waiter.set_result(None)
+                            break
                 raise
             finally:
                 if not self._closed:
                     self._acquired.remove(placeholder)
-                    self._acquired_per_host[key].remove(placeholder)
+                    self._drop_acquired_per_host(key, placeholder)
 
         self._acquired.add(proto)
         self._acquired_per_host[key].add(proto)
@@ -463,9 +473,7 @@ def _release_acquired(self, key, proto):
 
         try:
             self._acquired.remove(proto)
-            self._acquired_per_host[key].remove(proto)
-            if not self._acquired_per_host[key]:
-                del self._acquired_per_host[key]
+            self._drop_acquired_per_host(key, proto)
         except KeyError:  # pragma: no cover
             # this may be result of undetermenistic order of objects
             # finalization due garbage collection.
diff --git a/tests/test_client_functional.py b/tests/test_client_functional.py
index 2335de83065..66d1fe672b3 100644
--- a/tests/test_client_functional.py
+++ b/tests/test_client_functional.py
@@ -2288,3 +2288,63 @@ def close(self):
         assert resp.status == 200
     finally:
         yield from client.close()
+
+
+@asyncio.coroutine
+def test_error_in_performing_request(loop, ssl_ctx,
+                                     test_client, test_server):
+    @asyncio.coroutine
+    def handler(request):
+        return web.Response()
+
+    app = web.Application()
+    app.router.add_route('GET', '/', handler)
+
+    server = yield from test_server(app, ssl=ssl_ctx)
+
+    conn = aiohttp.TCPConnector(limit=1, loop=loop)
+    client = yield from test_client(server, connector=conn)
+
+    with pytest.raises(aiohttp.ClientConnectionError):
+        yield from client.get('/')
+
+    # second try should not hang
+    with pytest.raises(aiohttp.ClientConnectionError):
+        yield from client.get('/')
+
+
+@asyncio.coroutine
+def test_await_after_cancelling(loop, test_client):
+    @asyncio.coroutine
+    def handler(request):
+        return web.Response()
+
+    app = web.Application()
+    app.router.add_route('GET', '/', handler)
+
+    client = yield from test_client(app)
+
+    fut1 = create_future(loop)
+    fut2 = create_future(loop)
+
+    @asyncio.coroutine
+    def fetch1():
+        resp = yield from client.get('/')
+        assert resp.status == 200
+        fut1.set_result(None)
+        with pytest.raises(asyncio.CancelledError):
+            yield from fut2
+        resp.release()
+
+    @asyncio.coroutine
+    def fetch2():
+        yield from fut1
+        resp = yield from client.get('/')
+        assert resp.status == 200
+
+    @asyncio.coroutine
+    def canceller():
+        yield from fut1
+        fut2.cancel()
+
+    yield from asyncio.gather(fetch1(), fetch2(), canceller(), loop=loop)
diff --git a/tests/test_connector.py b/tests/test_connector.py
index b85d72d5d8b..c2cfab833fb 100644
--- a/tests/test_connector.py
+++ b/tests/test_connector.py
@@ -343,6 +343,28 @@ def test_release_close(loop):
     assert proto.close.called
 
 
+def test__drop_acquire_per_host1(loop):
+    conn = aiohttp.BaseConnector(loop=loop)
+    conn._drop_acquired_per_host(123, 456)
+    assert len(conn._acquired_per_host) == 0
+
+
+def test__drop_acquire_per_host2(loop):
+    conn = aiohttp.BaseConnector(loop=loop)
+    conn._acquired_per_host[123].add(456)
+    conn._drop_acquired_per_host(123, 456)
+    assert len(conn._acquired_per_host) == 0
+
+
+def test__drop_acquire_per_host3(loop):
+    conn = aiohttp.BaseConnector(loop=loop)
+    conn._acquired_per_host[123].add(456)
+    conn._acquired_per_host[123].add(789)
+    conn._drop_acquired_per_host(123, 456)
+    assert len(conn._acquired_per_host) == 1
+    assert conn._acquired_per_host[123] == {789}
+
+
 @asyncio.coroutine
 def test_tcp_connector_certificate_error(loop):
     req = ClientRequest('GET', URL('https://127.0.0.1:443'), loop=loop)