diff --git a/setup.py b/setup.py
index 8c0415e..63d8488 100755
--- a/setup.py
+++ b/setup.py
@@ -43,7 +43,7 @@ except:
 
 setuptools.setup(
         name='warcprox',
-        version='2.4.18',
+        version='2.4.21',
         description='WARC writing MITM HTTP/S proxy',
         url='https://github.com/internetarchive/warcprox',
         author='Noah Levitt',
diff --git a/tests/test_warcprox.py b/tests/test_warcprox.py
index b06043c..a420740 100755
--- a/tests/test_warcprox.py
+++ b/tests/test_warcprox.py
@@ -52,6 +52,7 @@ import mock
 import email.message
 import socketserver
 from concurrent import futures
+import urllib.parse
 
 try:
     import http.server as http_server
@@ -175,8 +176,10 @@ class _TestHttpRequestHandler(http_server.BaseHTTPRequestHandler):
     def build_response(self):
         m = re.match(r'^/([^/]+)/([^/]+)$', self.path)
         if m is not None:
-            special_header = 'warcprox-test-header: {}!'.format(m.group(1)).encode('utf-8')
-            payload = 'I am the warcprox test payload! {}!\n'.format(10*m.group(2)).encode('utf-8')
+            seg1 = urllib.parse.unquote(m.group(1))
+            seg2 = urllib.parse.unquote(m.group(2))
+            special_header = 'warcprox-test-header: {}!'.format(seg1).encode('utf-8')
+            payload = 'I am the warcprox test payload! {}!\n'.format(10*seg2).encode('utf-8')
             headers = (b'HTTP/1.1 200 OK\r\n'
                     +  b'Content-Type: text/plain\r\n'
                     +  special_header + b'\r\n'
@@ -1351,7 +1354,7 @@ def test_domain_data_soft_limit(
     warcprox_.proxy.remote_connection_pool.clear()
 
     # novel, pushes stats over the limit
-    url = 'https://muh.XN--Zz-2Ka.locALHOst:{}/z/~'.format(https_daemon.server_port)
+    url = 'https://muh.XN--Zz-2Ka.locALHOst:{}/z/%7E'.format(https_daemon.server_port)
     response = requests.get(
             url, proxies=archiving_proxies, headers=headers, stream=True,
             verify=False)
diff --git a/warcprox/controller.py b/warcprox/controller.py
index 9a2880e..84c3b93 100644
--- a/warcprox/controller.py
+++ b/warcprox/controller.py
@@ -37,6 +37,7 @@ import doublethink
 import importlib
 import queue
 import socket
+import os
 
 class Factory:
     @staticmethod
diff --git a/warcprox/dedup.py b/warcprox/dedup.py
index 9562fa5..0e09239 100644
--- a/warcprox/dedup.py
+++ b/warcprox/dedup.py
@@ -266,6 +266,9 @@ class CdxServerDedup(DedupDb):
         performance optimisation to handle that. limit < 0 is very inefficient
         in general. Maybe it could be configurable in the future.
 
+        Skip dedup for URLs with session params. These URLs are certainly
+        unique and highly volatile, we cannot dedup them.
+
         :param digest_key: b'sha1:<KEY-VALUE>' (prefix is optional).
             Example: b'sha1:B2LTWWPUOYAH7UIPQ7ZUPQ4VMBSVC36A'
         :param url: Target URL string
@@ -274,6 +277,8 @@ class CdxServerDedup(DedupDb):
         """
         u = url.decode("utf-8") if isinstance(url, bytes) else url
         try:
+            if any(s in u for s in ('JSESSIONID=', 'session=', 'sess=')):
+                return None
             result = self.http_pool.request('GET', self.cdx_url, fields=dict(
                 url=u, fl="timestamp,digest", filter="!mimetype:warc/revisit",
                 limit=-1))
diff --git a/warcprox/mitmproxy.py b/warcprox/mitmproxy.py
index 9bac478..8f86cd0 100644
--- a/warcprox/mitmproxy.py
+++ b/warcprox/mitmproxy.py
@@ -762,7 +762,7 @@ class PooledMitmProxy(PooledMixIn, MitmProxy):
         Abort active connections to remote servers to achieve prompt shutdown.
         '''
         self.shutting_down = True
-        for sock in self.remote_server_socks:
+        for sock in list(self.remote_server_socks):
             self.shutdown_request(sock)
 
 class SingleThreadedMitmProxy(http_server.HTTPServer):
@@ -780,7 +780,7 @@ class SingleThreadedMitmProxy(http_server.HTTPServer):
         self.bad_hostnames_ports_lock = RLock()
 
         self.remote_connection_pool = PoolManager(
-            num_pools=max((options.max_threads or 0) // 6, 400))
+            num_pools=max((options.max_threads or 0) // 6, 400), maxsize=6)
 
         if options.onion_tor_socks_proxy:
             try:
diff --git a/warcprox/writer.py b/warcprox/writer.py
index 730d606..cc44be2 100644
--- a/warcprox/writer.py
+++ b/warcprox/writer.py
@@ -167,14 +167,17 @@ class WarcWriter:
             if self.open_suffix == '':
                 try:
                     fcntl.lockf(self.f, fcntl.LOCK_UN)
-                except IOError as exc:
+                except Exception as exc:
                     self.logger.error(
                             'could not unlock file %s (%s)', self.path, exc)
-            self.f.close()
-            finalpath = os.path.sep.join(
-                    [self.directory, self.finalname])
-            os.rename(self.path, finalpath)
-
+            try:
+                self.f.close()
+                finalpath = os.path.sep.join(
+                        [self.directory, self.finalname])
+                os.rename(self.path, finalpath)
+            except Exception as exc:
+                self.logger.error(
+                    'could not close and rename file %s (%s)', self.path, exc)
             self.path = None
             self.f = None