diff --git a/python/test_requirements.txt b/python/test_requirements.txt
index 1170bb4d..43dc2869 100644
--- a/python/test_requirements.txt
+++ b/python/test_requirements.txt
@@ -1,6 +1,6 @@
 flake8>=2.0
 mock>=1.0.1
 nose>=1.3.0
-PyHamcrest>=1.7.2
+PyHamcrest>=1.8.0
 WebTest>=2.0.9
 
diff --git a/python/ycm/completers/cpp/clang_completer.py b/python/ycm/completers/cpp/clang_completer.py
index 1771dfad..0a835dfa 100644
--- a/python/ycm/completers/cpp/clang_completer.py
+++ b/python/ycm/completers/cpp/clang_completer.py
@@ -35,6 +35,7 @@ FILE_TOO_SHORT_MESSAGE = (
   'File is less than {0} lines long; not compiling.'.format(
     MIN_LINES_IN_FILE_TO_PARSE ) )
 NO_DIAGNOSTIC_MESSAGE = 'No diagnostic for current line!'
+PRAGMA_DIAG_TEXT_TO_IGNORE = '#pragma once in main file'
 
 
 class ClangCompleter( Completer ):
@@ -196,6 +197,7 @@ class ClangCompleter( Completer ):
       self.GetUnsavedFilesVector( request_data ),
       flags )
 
+    diagnostics = _FilterDiagnostics( diagnostics )
     self._diagnostic_store = DiagnosticsToDiagStructure( diagnostics )
     return [ ConvertToDiagnosticResponse( x ) for x in
              diagnostics[ : self._max_diagnostics_to_display ] ]
@@ -261,7 +263,7 @@ def ConvertCompletionData( completion_data ):
 
 
 def DiagnosticsToDiagStructure( diagnostics ):
-  structure = defaultdict(lambda : defaultdict(list))
+  structure = defaultdict( lambda : defaultdict( list ) )
   for diagnostic in diagnostics:
     structure[ diagnostic.filename_ ][ diagnostic.line_number_ ].append(
         diagnostic )
@@ -283,4 +285,14 @@ def ConvertToDiagnosticResponse( diagnostic ):
                                         diagnostic.text_,
                                         diagnostic.kind_ )
 
+def _FilterDiagnostics( diagnostics ):
+  # Clang has an annoying warning that shows up when we try to compile header
+  # files if the header has "#pragma once" inside it. The error is not
+  # legitimate because it shows up because libclang thinks we are compiling a
+  # source file instead of a header file.
+  #
+  # See our issue #216 and upstream bug:
+  #   http://llvm.org/bugs/show_bug.cgi?id=16686
+  return [ x for x in diagnostics if x.text_ != PRAGMA_DIAG_TEXT_TO_IGNORE ]
+
 
diff --git a/python/ycm/server/tests/diagnostics_test.py b/python/ycm/server/tests/diagnostics_test.py
index 6f1d199b..c0c87ace 100644
--- a/python/ycm/server/tests/diagnostics_test.py
+++ b/python/ycm/server/tests/diagnostics_test.py
@@ -23,7 +23,7 @@ from .test_utils import Setup, BuildRequest
 from webtest import TestApp
 from nose.tools import with_setup
 from hamcrest import ( assert_that, contains, contains_string, has_entries,
-                       has_entry )
+                       has_entry, empty )
 from .. import handlers
 import bottle
 
@@ -53,6 +53,29 @@ struct Foo {
                                  'line_num': 2,
                                  'column_num': 7 } ) ) )
 
+@with_setup( Setup )
+def Diagnostics_ClangCompleter_PragmaOnceWarningIgnored_test():
+  app = TestApp( handlers.app )
+  contents = """
+#pragma once
+
+struct Foo {
+  int x;
+  int y;
+  int c;
+  int d;
+};
+"""
+
+  event_data = BuildRequest( compilation_flags = ['-x', 'c++'],
+                             event_name = 'FileReadyToParse',
+                             contents = contents,
+                             filepath = '/foo.h',
+                             filetype = 'cpp' )
+
+  response = app.post_json( '/event_notification', event_data )
+  assert_that( response.body, empty() )
+
 
 @with_setup( Setup )
 def GetDetailedDiagnostic_ClangCompleter_Works_test():