diff --git a/app/controllers/api/base_controller.rb b/app/controllers/api/base_controller.rb
index de8fff30e..33df75b37 100644
--- a/app/controllers/api/base_controller.rb
+++ b/app/controllers/api/base_controller.rb
@@ -36,6 +36,14 @@ class Api::BaseController < ApplicationController
     render json: { error: 'This action is not allowed' }, status: 403
   end
 
+  rescue_from Mastodon::RaceConditionError do
+    render json: { error: 'There was a temporary problem serving your request, please try again' }, status: 503
+  end
+
+  rescue_from ActionController::ParameterMissing do |e|
+    render json: { error: e.to_s }, status: 400
+  end
+
   def doorkeeper_unauthorized_render_options(error: nil)
     { json: { error: (error.try(:description) || 'Not authorized') } }
   end
diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 1caaa20f7..5b343a276 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -21,11 +21,13 @@ class ApplicationController < ActionController::Base
   helper_method :whitelist_mode?
 
   rescue_from ActionController::RoutingError, with: :not_found
-  rescue_from ActiveRecord::RecordNotFound, with: :not_found
   rescue_from ActionController::InvalidAuthenticityToken, with: :unprocessable_entity
   rescue_from ActionController::UnknownFormat, with: :not_acceptable
+  rescue_from ActionController::ParameterMissing, with: :bad_request
+  rescue_from ActiveRecord::RecordNotFound, with: :not_found
   rescue_from Mastodon::NotPermittedError, with: :forbidden
   rescue_from HTTP::Error, OpenSSL::SSL::SSLError, with: :internal_server_error
+  rescue_from Mastodon::RaceConditionError, with: :service_unavailable
 
   before_action :store_current_location, except: :raise_not_found, unless: :devise_controller?
   before_action :require_functional!, if: :user_signed_in?
@@ -96,10 +98,18 @@ class ApplicationController < ActionController::Base
     respond_with_error(406)
   end
 
+  def bad_request
+    respond_with_error(400)
+  end
+
   def internal_server_error
     respond_with_error(500)
   end
 
+  def service_unavailable
+    respond_with_error(503)
+  end
+
   def single_user_mode?
     @single_user_mode ||= Rails.configuration.x.single_user_mode && Account.where('id > 0').exists?
   end
diff --git a/app/views/errors/400.html.haml b/app/views/errors/400.html.haml
new file mode 100644
index 000000000..11fbdd40c
--- /dev/null
+++ b/app/views/errors/400.html.haml
@@ -0,0 +1,5 @@
+- content_for :page_title do
+  = t('errors.400')
+
+- content_for :content do
+  = t('errors.400')
diff --git a/app/views/errors/406.html.haml b/app/views/errors/406.html.haml
new file mode 100644
index 000000000..0ef815df3
--- /dev/null
+++ b/app/views/errors/406.html.haml
@@ -0,0 +1,5 @@
+- content_for :page_title do
+  = t('errors.406')
+
+- content_for :content do
+  = t('errors.406')
diff --git a/app/views/errors/503.html.haml b/app/views/errors/503.html.haml
new file mode 100644
index 000000000..b0c895aa5
--- /dev/null
+++ b/app/views/errors/503.html.haml
@@ -0,0 +1,5 @@
+- content_for :page_title do
+  = t('errors.503')
+
+- content_for :content do
+  = t('errors.503')
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 2f601f274..892d13c72 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -652,8 +652,10 @@ en:
   domain_validator:
     invalid_domain: is not a valid domain name
   errors:
+    '400': The request you submitted was invalid or malformed.
     '403': You don't have permission to view this page.
     '404': The page you are looking for isn't here.
+    '406': This page is not available in the requested format.
     '410': The page you were looking for doesn't exist here anymore.
     '422':
       content: Security verification failed. Are you blocking cookies?
@@ -662,6 +664,7 @@ en:
     '500':
       content: We're sorry, but something went wrong on our end.
       title: This page is not correct
+    '503': The page could not be served due to a temporary server failure.
     noscript_html: To use the Mastodon web application, please enable JavaScript. Alternatively, try one of the <a href="%{apps_path}">native apps</a> for Mastodon for your platform.
   existing_username_validator:
     not_found: could not find a local user with that username
diff --git a/spec/controllers/settings/two_factor_authentication/confirmations_controller_spec.rb b/spec/controllers/settings/two_factor_authentication/confirmations_controller_spec.rb
index 478f24585..2222a7559 100644
--- a/spec/controllers/settings/two_factor_authentication/confirmations_controller_spec.rb
+++ b/spec/controllers/settings/two_factor_authentication/confirmations_controller_spec.rb
@@ -50,7 +50,8 @@ describe Settings::TwoFactorAuthentication::ConfirmationsController do
 
       describe 'when form_two_factor_confirmation parameter is not provided' do
         it 'raises ActionController::ParameterMissing' do
-          expect { post :create, params: {} }.to raise_error(ActionController::ParameterMissing)
+          post :create, params: {}
+          expect(response).to have_http_status(400)
         end
       end
 
diff --git a/spec/controllers/settings/two_factor_authentications_controller_spec.rb b/spec/controllers/settings/two_factor_authentications_controller_spec.rb
index 9f27222ad..f7c628756 100644
--- a/spec/controllers/settings/two_factor_authentications_controller_spec.rb
+++ b/spec/controllers/settings/two_factor_authentications_controller_spec.rb
@@ -112,7 +112,8 @@ describe Settings::TwoFactorAuthenticationsController do
       end
 
       it 'raises ActionController::ParameterMissing if code is missing' do
-        expect { post :destroy }.to raise_error(ActionController::ParameterMissing)
+        post :destroy
+        expect(response).to have_http_status(400)
       end
     end