diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -904,6 +904,7 @@
     'DiffusionLowLevelResolveRefsQuery' => 'applications/diffusion/query/lowlevel/DiffusionLowLevelResolveRefsQuery.php',
     'DiffusionMercurialBlameQuery' => 'applications/diffusion/query/blame/DiffusionMercurialBlameQuery.php',
     'DiffusionMercurialCommandEngine' => 'applications/diffusion/protocol/DiffusionMercurialCommandEngine.php',
+    'DiffusionMercurialCommandEngineTests' => 'applications/diffusion/protocol/__tests__/DiffusionMercurialCommandEngineTests.php',
     'DiffusionMercurialFileContentQuery' => 'applications/diffusion/query/filecontent/DiffusionMercurialFileContentQuery.php',
     'DiffusionMercurialFlagInjectionException' => 'applications/diffusion/exception/DiffusionMercurialFlagInjectionException.php',
     'DiffusionMercurialRawDiffQuery' => 'applications/diffusion/query/rawdiff/DiffusionMercurialRawDiffQuery.php',
@@ -7041,6 +7042,7 @@
     'DiffusionLowLevelResolveRefsQuery' => 'DiffusionLowLevelQuery',
     'DiffusionMercurialBlameQuery' => 'DiffusionBlameQuery',
     'DiffusionMercurialCommandEngine' => 'DiffusionCommandEngine',
+    'DiffusionMercurialCommandEngineTests' => 'PhabricatorTestCase',
     'DiffusionMercurialFileContentQuery' => 'DiffusionFileContentQuery',
     'DiffusionMercurialFlagInjectionException' => 'Exception',
     'DiffusionMercurialRawDiffQuery' => 'DiffusionRawDiffQuery',
diff --git a/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php b/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php
--- a/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php
+++ b/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php
@@ -166,6 +166,15 @@
 
     $last = null;
     foreach (array_reverse($lines) as $line) {
+      // In the event additional log output is included in future mercurial
+      // updates, if the line does not contain any semi-colon then log it and
+      // ignore it.
+      if (strpos($line, ';') === false) {
+        phlog(pht(
+          'Unexpected output from mercurial "log --debug" command: %s',
+          $line));
+        continue;
+      }
       list($hash, $parents) = explode(';', $line);
       $parents = trim($parents);
       if (!$parents) {
diff --git a/src/applications/diffusion/protocol/DiffusionMercurialCommandEngine.php b/src/applications/diffusion/protocol/DiffusionMercurialCommandEngine.php
--- a/src/applications/diffusion/protocol/DiffusionMercurialCommandEngine.php
+++ b/src/applications/diffusion/protocol/DiffusionMercurialCommandEngine.php
@@ -71,12 +71,36 @@
     //
     // Separately, it may fail to write to a different branch cache, and may
     // encounter issues reading the branch cache.
+    //
+    // When Mercurial repositories are hosted on external systems with
+    // multi-user environments it's possible that the branch cache is computed
+    // on a revision which does not end up being published. When this happens it
+    // will recompute the cache but also print out "invalid branch cache".
+    //
+    // https://www.mercurial-scm.org/pipermail/mercurial/2014-June/047239.html
+    //
+    // When observing a repository which uses largefiles, the debug output may
+    // also contain extraneous output about largefile changes.
+    //
+    // At some point Mercurial added/improved support for pager used when
+    // command output is large. It includes printing out debug information that
+    // the pager is being started for a command. This seems to happen despite
+    // the output of the command being piped/read from another process.
+    //
+    // When printing color output Mercurial may run into some issue with the
+    // terminal info. This should never happen in Phabricator since color
+    // output should be turned off, however in the event it shows up we should
+    // filter it out anyways.
 
     $ignore = array(
       'ignoring untrusted configuration option',
       "couldn't write revision branch cache:",
       "couldn't write branch cache:",
       'invalid branchheads cache',
+      'invalid branch cache',
+      'updated patterns: .hglf',
+      'starting pager for command',
+      'no terminfo entry for',
     );
 
     foreach ($ignore as $key => $pattern) {
diff --git a/src/applications/diffusion/protocol/__tests__/DiffusionMercurialCommandEngineTests.php b/src/applications/diffusion/protocol/__tests__/DiffusionMercurialCommandEngineTests.php
new file mode 100644
--- /dev/null
+++ b/src/applications/diffusion/protocol/__tests__/DiffusionMercurialCommandEngineTests.php
@@ -0,0 +1,42 @@
+<?php
+
+final class DiffusionMercurialCommandEngineTests extends PhabricatorTestCase {
+
+  public function testFilteringDebugOutput() {
+    // Output that should be filtered out from the results
+    $output =
+      "ignoring untrusted configuration option\n".
+      "couldn't write revision branch cache:\n".
+      "couldn't write branch cache: blah blah blah\n".
+      "invalid branchheads cache\n".
+      "invalid branch cache (served): tip differs\n".
+      "starting pager for command 'log'\n".
+      "updated patterns: ".
+        ".hglf/project/src/a/b/c/SomeClass.java, ".
+        "project/src/a/b/c/SomeClass.java\n".
+      "no terminfo entry for sitm\n";
+
+    $filtered_output =
+      DiffusionMercurialCommandEngine::filterMercurialDebugOutput($output);
+
+    $this->assertEqual('', $filtered_output);
+
+    // The output that should make it through the filtering
+    $output =
+      "0b33a9e5ceedba14b03214f743957357d7bb46a9;694".
+        ":8b39f63eb209dd2bdfd4bd3d0721a9e38d75a6d3".
+        "-1:0000000000000000000000000000000000000000\n".
+      "8b39f63eb209dd2bdfd4bd3d0721a9e38d75a6d3;693".
+        ":165bce9ce4ccc97024ba19ed5a22f6a066fa6844".
+        "-1:0000000000000000000000000000000000000000\n".
+      "165bce9ce4ccc97024ba19ed5a22f6a066fa6844;692:".
+        "2337bc9e3cf212b3b386b5197801b1c81db64920".
+        "-1:0000000000000000000000000000000000000000\n";
+
+    $filtered_output =
+      DiffusionMercurialCommandEngine::filterMercurialDebugOutput($output);
+
+    $this->assertEqual($output, $filtered_output);
+  }
+
+}