diff --git a/src/__tests__/PhutilLibraryTestCase.php b/src/__tests__/PhutilLibraryTestCase.php
--- a/src/__tests__/PhutilLibraryTestCase.php
+++ b/src/__tests__/PhutilLibraryTestCase.php
@@ -40,6 +40,67 @@
   }
 
   /**
+   * This is more of an acceptance test case instead of a unit test. It verifies
+   * that methods in subclasses have the same visibility as the method in the
+   * parent class.
+   */
+  public function testMethodVisibility() {
+    $symbols = id(new PhutilSymbolLoader())
+      ->selectSymbolsWithoutLoading();
+
+    $classes = array();
+    foreach ($symbols as $symbol) {
+      if ($symbol['type'] == 'class') {
+        $classes[$symbol['name']] = new ReflectionClass($symbol['name']);
+      }
+    }
+
+    $failures = array();
+
+    foreach ($classes as $class_name => $class) {
+      $parents = array();
+      $parent = $class;
+      while ($parent = $parent->getParentClass()) {
+        $parents[] = $parent;
+      }
+
+      $interfaces = $class->getInterfaces();
+
+      foreach ($class->getMethods() as $method) {
+        $visibility = $this->getVisibility($method);
+        $method_name = $method->getName();
+
+        foreach (array_merge($parents, $interfaces) as $extends) {
+          if ($extends->hasMethod($method_name)) {
+            $xmethod = $extends->getMethod($method_name);
+            $xvisibility = $this->getVisibility($xmethod);
+
+            if ($xvisibility != $visibility) {
+              $failures[] = pht(
+                'Class "%s" implements method "%s" with the wrong visibility. '.
+                'The method has visibility "%s", but it is defined in parent '.
+                '"%s" with visibility "%s". In Phabricator, a method which '.
+                'overrides another must always have the same visibility.',
+                $class_name,
+                $method_name,
+                $visibility,
+                $extends->getName(),
+                $xvisibility);
+            }
+
+            // We found a declaration somewhere, so stop looking.
+            break;
+          }
+        }
+      }
+    }
+
+    $this->assertTrue(
+      empty($failures),
+      "\n\n".implode("\n\n", $failures));
+  }
+
+  /**
    * Get the root directory for the library currently being tested.
    */
   protected function getLibraryRoot() {
@@ -47,4 +108,14 @@
     return phutil_get_library_root_for_path($caller);
   }
 
+  private function getVisibility(ReflectionMethod $method) {
+    if ($method->isPrivate()) {
+      return 'private';
+    } else if ($method->isProtected()) {
+      return 'protected';
+    } else {
+      return 'public';
+    }
+  }
+
 }