diff --git a/src/infrastructure/util/PhabricatorGlobalLock.php b/src/infrastructure/util/PhabricatorGlobalLock.php
--- a/src/infrastructure/util/PhabricatorGlobalLock.php
+++ b/src/infrastructure/util/PhabricatorGlobalLock.php
@@ -118,6 +118,28 @@
     self::$pool = array();
   }
 
+  public static function newConnection() {
+    // NOTE: Use of the "repository" database is somewhat arbitrary, mostly
+    // because the first client of locks was the repository daemons.
+
+    // We must always use the same database for all locks, because different
+    // databases may be on different hosts if the database is partitioned.
+
+    // However, we don't access any tables so we could use any valid database.
+    // We could build a database-free connection instead, but that's kind of
+    // messy and unusual.
+
+    $dao = new PhabricatorRepository();
+
+    // NOTE: Using "force_new" to make sure each lock is on its own connection.
+
+    // See T13627. This is critically important in versions of MySQL older
+    // than MySQL 5.7, because they can not hold more than one lock per
+    // connection simultaneously.
+
+    return $dao->establishConnection('w', $force_new = true);
+  }
+
 /* -(  Implementation  )----------------------------------------------------- */
 
   protected function doLock($wait) {
@@ -135,18 +157,7 @@
     }
 
     if (!$conn) {
-      // NOTE: Using the 'repository' database somewhat arbitrarily, mostly
-      // because the first client of locks is the repository daemons. We must
-      // always use the same database for all locks, but don't access any
-      // tables so we could use any valid database. We could build a
-      // database-free connection instead, but that's kind of messy and we
-      // might forget about it in the future if we vertically partition the
-      // application.
-      $dao = new PhabricatorRepository();
-
-      // NOTE: Using "force_new" to make sure each lock is on its own
-      // connection.
-      $conn = $dao->establishConnection('w', $force_new = true);
+      $conn = self::newConnection();
     }
 
     // See T13627. We must never hold more than one lock per connection, so
@@ -189,7 +200,12 @@
       // example, this can happen if there are a large number of webhook tasks
       // in the queue.
 
-      self::$pool[] = $conn;
+      // See T13627. If this is an external connection, don't put it into
+      // the shared connection pool.
+
+      if (!$this->externalConnection) {
+        self::$pool[] = $conn;
+      }
 
       throw id(new PhutilLockException($lock_name))
         ->setHint($this->newHint($lock_name, $wait));
diff --git a/src/infrastructure/util/__tests__/PhabricatorGlobalLockTestCase.php b/src/infrastructure/util/__tests__/PhabricatorGlobalLockTestCase.php
--- a/src/infrastructure/util/__tests__/PhabricatorGlobalLockTestCase.php
+++ b/src/infrastructure/util/__tests__/PhabricatorGlobalLockTestCase.php
@@ -37,8 +37,7 @@
   }
 
   public function testConnectionPoolWithSpecificConnection() {
-    $conn = id(new HarbormasterScratchTable())
-      ->establishConnection('w');
+    $conn = PhabricatorGlobalLock::newConnection();
 
     PhabricatorGlobalLock::clearConnectionPool();
 
@@ -86,8 +85,7 @@
   }
 
   public function testExternalConnectionMutationScope() {
-    $conn = id(new HarbormasterScratchTable())
-      ->establishConnection('w');
+    $conn = PhabricatorGlobalLock::newConnection();
 
     $lock_name = $this->newLockName();
     $lock = PhabricatorGlobalLock::newLock($lock_name);
@@ -110,8 +108,7 @@
   }
 
   public function testMultipleLocks() {
-    $conn = id(new HarbormasterScratchTable())
-      ->establishConnection('w');
+    $conn = PhabricatorGlobalLock::newConnection();
 
     PhabricatorGlobalLock::clearConnectionPool();
 
@@ -143,8 +140,88 @@
       pht('Expect multiple locks on the same connection to fail.'));
   }
 
+  public function testPoolReleaseOnFailure() {
+    $conn = PhabricatorGlobalLock::newConnection();
+    $lock_name = $this->newLockName();
+
+    PhabricatorGlobalLock::clearConnectionPool();
+
+    $this->assertEqual(
+      0,
+      PhabricatorGlobalLock::getConnectionPoolSize(),
+      pht('Clear Connection Pool'));
+
+    $lock = PhabricatorGlobalLock::newLock($lock_name);
+
+    // NOTE: We're cheating here, since there's a global registry of locks
+    // for the process that we have to bypass. In the real world, this lock
+    // would have to be held by some external process. To simplify this
+    // test case, just use a raw "GET_LOCK()" call to hold the lock.
+
+    $raw_conn = PhabricatorGlobalLock::newConnection();
+    $raw_name = $lock->getName();
+
+    $row = queryfx_one(
+      $raw_conn,
+      'SELECT GET_LOCK(%s, %f)',
+      $raw_name,
+      0);
+    $this->assertTrue((bool)head($row), pht('Establish Raw Lock'));
+
+    $this->assertEqual(
+      0,
+      PhabricatorGlobalLock::getConnectionPoolSize(),
+      pht('Connection Pool with Held Lock'));
+
+    // We expect this sequence to establish a new connection, fail to acquire
+    // the lock, then put the connection in the connection pool. After the
+    // first cycle, the connection should be reused.
+
+    for ($ii = 0; $ii < 3; $ii++) {
+      $this->tryHeldLock($lock_name);
+      $this->assertEqual(
+        1,
+        PhabricatorGlobalLock::getConnectionPoolSize(),
+        pht('Connection Pool After Lock Failure'));
+    }
+
+    PhabricatorGlobalLock::clearConnectionPool();
+
+    // Now, do the same thing with an external connection. This connection
+    // should not be put into the pool! See T13627.
+
+    for ($ii = 0; $ii < 3; $ii++) {
+      $this->tryHeldLock($lock_name, $conn);
+      $this->assertEqual(
+        0,
+        PhabricatorGlobalLock::getConnectionPoolSize(),
+        pht('Connection Pool After External Lock Failure'));
+    }
+  }
+
   private function newLockName() {
     return 'testlock-'.Filesystem::readRandomCharacters(16);
   }
 
+  private function tryHeldLock(
+    $lock_name,
+    AphrontDatabaseConnection $conn = null) {
+
+    $lock = PhabricatorGlobalLock::newLock($lock_name);
+
+    if ($conn) {
+      $lock->setExternalConnection($conn);
+    }
+
+    $caught = null;
+    try {
+      $lock->lock(0);
+    } catch (PhutilLockException $ex) {
+      $caught = $ex;
+    }
+
+    $this->assertTrue($caught instanceof PhutilLockException);
+  }
+
+
 }