Use verb form of "fallback"

"Fallback" is a noun, whereas "fall back" is a verb.

actionmailbox/test/dummy/config/environments/production.rb

@@ -26,7 +26,7 @@ Rails.application.configure do
   # Compress CSS using a preprocessor.
   # config.assets.css_compressor = :sass
 
-  # Do not fallback to assets pipeline if a precompiled asset is missed.
+  # Do not fall back to assets pipeline if a precompiled asset is missed.
   config.assets.compile = false
 
   # Enable serving of images, stylesheets, and JavaScripts from an asset server.

actionpack/lib/action_controller/metal/params_wrapper.rb

@@ -72,7 +72,7 @@ module ActionController
   #
   # will try to check if +Admin::User+ or +User+ model exists, and use it to
   # determine the wrapper key respectively. If both models don't exist,
-  # it will then fallback to use +user+ as the key.
+  # it will then fall back to use +user+ as the key.
   #
   # To disable this functionality for a controller:
   #

actionpack/lib/action_controller/metal/redirecting.rb

@@ -154,7 +154,7 @@ module ActionController
     public :_compute_redirect_to_location
 
     # Verifies the passed +location+ is an internal URL that's safe to redirect to and returns it, or nil if not.
-    # Useful to wrap a params provided redirect URL and fallback to an alternate URL to redirect to:
+    # Useful to wrap a params provided redirect URL and fall back to an alternate URL to redirect to:
     #
     #   redirect_to url_from(params[:redirect_url]) || root_url
     #

actionpack/lib/action_dispatch/http/mime_negotiation.rb

@@ -118,7 +118,7 @@ module ActionDispatch
       # Sets the \formats by string extensions. This differs from #format= by allowing you
       # to set multiple, ordered formats, which is useful when you want to have a fallback.
       #
-      # In this example, the +:iphone+ format will be used if it's available, otherwise it'll fallback
+      # In this example, the +:iphone+ format will be used if it's available, otherwise it'll fall back
       # to the +:html+ format.
       #
       #   class ApplicationController < ActionController::Base

actionpack/lib/action_dispatch/http/request.rb

@@ -201,7 +201,7 @@ module ActionDispatch
     # more information.
     #
     # For debugging purposes, when called with arguments this method will
-    # fallback to Object#method
+    # fall back to Object#method
     def method(*args)
       if args.empty?
         @method ||= check_method(

actiontext/test/dummy/config/environments/production.rb

@@ -26,7 +26,7 @@ Rails.application.configure do
   # Compress CSS using a preprocessor.
   # config.assets.css_compressor = :sass
 
-  # Do not fallback to assets pipeline if a precompiled asset is missed.
+  # Do not fall back to assets pipeline if a precompiled asset is missed.
   config.assets.compile = false
 
   # Enable serving of images, stylesheets, and JavaScripts from an asset server.

actionview/lib/action_view/layouts.rb

@@ -152,7 +152,7 @@ module ActionView
   # The template will be looked always in <tt>app/views/layouts/</tt> folder. But you can point
   # <tt>layouts</tt> folder direct also. <tt>layout "layouts/demo"</tt> is the same as <tt>layout "demo"</tt>.
   #
-  # Setting the layout to +nil+ forces it to be looked up in the filesystem and fallbacks to the parent behavior if none exists.
+  # Setting the layout to +nil+ forces it to be looked up in the filesystem and falls back to the parent behavior if none exists.
   # Setting it to +nil+ is useful to re-enable template lookup overriding a previous configuration set in the parent:
   #
   #     class ApplicationController < ActionController::Base
@@ -164,7 +164,7 @@ module ActionView
   #     end
   #
   #     class CommentsController < ApplicationController
-  #       # Will search for "comments" layout and fallback "application" layout
+  #       # Will search for "comments" layout and fall back to "application" layout
   #       layout nil
   #     end
   #

activerecord/lib/active_record/associations.rb

@@ -1050,7 +1050,7 @@ module ActiveRecord
       # query per addressable type.
       # For example, if all the addressables are either of class Person or Company, then a total
       # of 3 queries will be executed. The list of addressable types to load is determined on
-      # the back of the addresses loaded. This is not supported if Active Record has to fallback
+      # the back of the addresses loaded. This is not supported if Active Record has to fall back
       # to the previous implementation of eager loading and will raise ActiveRecord::EagerLoadPolymorphicError.
       # The reason is that the parent model's type is a column value so its corresponding table
       # name cannot be put in the +FROM+/+JOIN+ clauses of that query.

activestorage/test/dummy/config/environments/production.rb

@@ -26,7 +26,7 @@ Rails.application.configure do
   # Compress CSS using a preprocessor.
   # config.assets.css_compressor = :sass
 
-  # Do not fallback to assets pipeline if a precompiled asset is missed.
+  # Do not fall back to assets pipeline if a precompiled asset is missed.
   config.assets.compile = false
 
   # Enable serving of images, stylesheets, and JavaScripts from an asset server.

activesupport/lib/active_support/core_ext/string/indent.rb

@@ -24,7 +24,7 @@ class String
   #
   # The second argument, +indent_string+, specifies which indent string to
   # use. The default is +nil+, which tells the method to make a guess by
-  # peeking at the first indented line, and fallback to a space if there is
+  # peeking at the first indented line, and fall back to a space if there is
   # none.
   #
   #   "  foo".indent(2)        # => "    foo"

railties/lib/rails/generators.rb

@@ -90,7 +90,7 @@ module Rails
       end
 
       # Hold configured generators fallbacks. If a plugin developer wants a
-      # generator group to fallback to another group in case of missing generators,
+      # generator group to fall back to another group in case of missing generators,
       # they can add a fallback.
       #
       # For example, shoulda is considered a test_framework and is an extension

railties/lib/rails/generators/rails/app/templates/config/environments/production.rb.tt

@@ -29,7 +29,7 @@ Rails.application.configure do
   # Compress CSS using a preprocessor.
   # config.assets.css_compressor = :sass
 
-  # Do not fallback to assets pipeline if a precompiled asset is missed.
+  # Do not fall back to assets pipeline if a precompiled asset is missed.
   config.assets.compile = false
 
   <%- end -%>