diff --git a/pyflakes/checker.py b/pyflakes/checker.py
index <HASH>..<HASH> 100644
--- a/pyflakes/checker.py
+++ b/pyflakes/checker.py
@@ -163,7 +163,8 @@ class ModuleScope(Scope):
     pass
 
 
-# Globally defined names which are not attributes of the __builtin__ module.
+# Globally defined names which are not attributes of the __builtin__ module, or
+# are only present on some platforms.
 _MAGIC_GLOBALS = ['__file__', '__builtins__', 'WindowsError']
Adjust the comment about _MAGIC_GLOBALS, explaining that it also papers over cross-platform __builtin__ differences.