diff --git a/tools/gn/args.cc b/tools/gn/args.cc
index 1034ea1..496ba18 100644
--- a/tools/gn/args.cc
+++ b/tools/gn/args.cc
@@ -194,25 +194,12 @@
   if (all_overrides.empty())
     return true;
 
-  // Get a list of all possible overrides for help with error finding.
-  //
-  // It might be nice to do edit distance checks to see if we can find one close
-  // to what you typed.
-  std::string all_declared_str;
-  for (const auto& map_pair : declared_arguments_per_toolchain_) {
-    for (const auto& cur_str : map_pair.second) {
-      if (!all_declared_str.empty())
-        all_declared_str += ", ";
-      all_declared_str += cur_str.first.as_string();
-    }
-  }
-
   *err = Err(
       all_overrides.begin()->second.origin(), "Build argument has no effect.",
       "The variable \"" + all_overrides.begin()->first.as_string() +
           "\" was set as a build argument\nbut never appeared in a " +
-          "declare_args() block in any buildfile.\n\nPossible arguments: " +
-          all_declared_str);
+          "declare_args() block in any buildfile.\n\n"
+          "To view possible args, run \"gn args --list <builddir>\"");
   return false;
 }
 
diff --git a/tools/gn/docs/reference.md b/tools/gn/docs/reference.md
index 600daea..7b345d7 100644
--- a/tools/gn/docs/reference.md
+++ b/tools/gn/docs/reference.md
@@ -70,7 +70,29 @@
 
 
 ```
-## **\--markdown**: write the output in the Markdown format.
+## **\--fail-on-unused-args**: Treat unused build args as fatal errors.
+
+```
+  If you set a value in a build's "gn args" and never use it in the
+  build (in a declare_args() block), GN will normally print an error
+  but not fail the build.
+
+  In many cases engineers would use build args to enable or disable
+  features that would sometimes get removed. It would by annoying to
+  block work for typically benign problems. In Chrome in particular,
+  flags might be configured for build bots in a separate infrastructure
+  repository, or a declare_args block might be changed in a third party
+  repository. Treating these errors as blocking forced complex multi-
+  way patches to land what would otherwise be simple changes.
+
+  In some cases, such concerns are not as important, and a mismatch
+  in build flags between the invoker of the build and the build files
+  represents a critical mismatch that should be immediately fixed. Such
+  users can set this flag to force GN to fail in that case.
+
+
+```
+## **\--markdown**: Write help output in the Markdown format.
 
 ## **\--[no]color**: Forces colored output on or off.
 
@@ -5925,7 +5947,8 @@
 **  \--args**: Specifies build arguments overrides.
 **  \--color**: Force colored output.
 **  \--dotfile**: Override the name of the ".gn" file.
-**  \--markdown**: write the output in the Markdown format.
+**  \--fail-on-unused-args**: Treat unused build args as fatal errors.
+**  \--markdown**: Write help output in the Markdown format.
 **  \--nocolor**: Force non-colored output.
 **  -q**: Quiet mode. Don't print output on success.
 **  \--root**: Explicitly specify source root.
diff --git a/tools/gn/setup.cc b/tools/gn/setup.cc
index 384a2af..7f8e286 100644
--- a/tools/gn/setup.cc
+++ b/tools/gn/setup.cc
@@ -332,9 +332,13 @@
     }
 
     if (!build_settings_.build_args().VerifyAllOverridesUsed(&err)) {
-      // TODO(brettw) implement a system of warnings. Until we have a better
-      // system, print the error but don't return failure.
+      // TODO(brettw) implement a system to have a different marker for
+      // warnings. Until we have a better system, print the error but don't
+      // return failure unless requested on the command line.
       err.PrintToStdout();
+      if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+              switches::kFailOnUnusedArgs))
+        return false;
       return true;
     }
   }
diff --git a/tools/gn/switches.cc b/tools/gn/switches.cc
index df167b3..45b25e6 100644
--- a/tools/gn/switches.cc
+++ b/tools/gn/switches.cc
@@ -69,11 +69,34 @@
     "  Note that this interacts with \"--root\" in a possibly incorrect way.\n"
     "  It would be nice to test the edge cases and document or fix.\n";
 
+const char kFailOnUnusedArgs[] = "fail-on-unused-args";
+const char kFailOnUnusedArgs_HelpShort[] =
+    "--fail-on-unused-args: Treat unused build args as fatal errors.";
+const char kFailOnUnusedArgs_Help[] =
+    "--fail-on-unused-args: Treat unused build args as fatal errors.\n"
+    "\n"
+    "  If you set a value in a build's \"gn args\" and never use it in the\n"
+    "  build (in a declare_args() block), GN will normally print an error\n"
+    "  but not fail the build.\n"
+    "\n"
+    "  In many cases engineers would use build args to enable or disable\n"
+    "  features that would sometimes get removed. It would by annoying to\n"
+    "  block work for typically benign problems. In Chrome in particular,\n"
+    "  flags might be configured for build bots in a separate infrastructure\n"
+    "  repository, or a declare_args block might be changed in a third party\n"
+    "  repository. Treating these errors as blocking forced complex multi-\n"
+    "  way patches to land what would otherwise be simple changes.\n"
+    "\n"
+    "  In some cases, such concerns are not as important, and a mismatch\n"
+    "  in build flags between the invoker of the build and the build files\n"
+    "  represents a critical mismatch that should be immediately fixed. Such\n"
+    "  users can set this flag to force GN to fail in that case.\n";
+
 const char kMarkdown[] = "markdown";
 const char kMarkdown_HelpShort[] =
-    "--markdown: write the output in the Markdown format.";
+    "--markdown: Write help output in the Markdown format.";
 const char kMarkdown_Help[] =
-    "--markdown: write the output in the Markdown format.\n";
+    "--markdown: Write help output in the Markdown format.\n";
 
 const char kNoColor[] = "nocolor";
 const char kNoColor_HelpShort[] =
@@ -223,6 +246,7 @@
     INSERT_VARIABLE(Args)
     INSERT_VARIABLE(Color)
     INSERT_VARIABLE(Dotfile)
+    INSERT_VARIABLE(FailOnUnusedArgs)
     INSERT_VARIABLE(Markdown)
     INSERT_VARIABLE(NoColor)
     INSERT_VARIABLE(Root)
diff --git a/tools/gn/switches.h b/tools/gn/switches.h
index 3dc5b9f..cebb19c 100644
--- a/tools/gn/switches.h
+++ b/tools/gn/switches.h
@@ -40,6 +40,10 @@
 extern const char kDotfile_HelpShort[];
 extern const char kDotfile_Help[];
 
+extern const char kFailOnUnusedArgs[];
+extern const char kFailOnUnusedArgs_HelpShort[];
+extern const char kFailOnUnusedArgs_Help[];
+
 extern const char kMarkdown[];
 extern const char kMarkdown_HelpShort[];
 extern const char kMarkdown_Help[];