diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,12 @@
 ## MASTER
+## Dotenv 0.10.1.0
+### Added
+* Short `-x` for `--example` flag
+
+### Modified
+* Fix `union`/`unionBy` bug, refactor .env.example check, and improve error
+message (Kudos to @pbrisbin)
+
 ## Dotenv 0.10.0.1
 ### Modified
 * Modify docs.
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-[![Build Status](https://github.com/stackbuilders/dotenv-hs/actions/workflows/build-and-test.yml/badge.svg)](https://github.com/stackbuilders/dotenv-hs/actions/workflows/build-and-test.yml) [![Hackage](https://img.shields.io/hackage/v/dotenv.svg)](http://hackage.haskell.org/package/dotenv)
+[![Build Status](https://github.com/stackbuilders/dotenv-hs/actions/workflows/build.yml/badge.svg)](https://github.com/stackbuilders/dotenv-hs/actions/workflows/build.yml)[![Hackage](https://img.shields.io/hackage/v/dotenv.svg)](http://hackage.haskell.org/package/dotenv)
 
 # Dotenv files for Haskell
 
@@ -161,7 +161,7 @@
 This will fail:
 ```shell
 $ dotenv -f .env --example .env.example "myprogram --myflag myargument"
-> dotenv: Missing env vars! Please, check (this/these) var(s) (is/are) set: BAR
+> dotenv: The following variables are present in .env.example, but not set in the current environment, or .env: BAR
 ```
 
 This will succeed:
diff --git a/app/Main.hs b/app/Main.hs
--- a/app/Main.hs
+++ b/app/Main.hs
@@ -62,6 +62,7 @@
 
      <*> many (strOption (
                   long "example"
+                  <> short 'x'
                   <> metavar "DOTENV_EXAMPLE"
                   <> help "File to read for needed environmental variables" ))
 
diff --git a/dotenv.cabal b/dotenv.cabal
--- a/dotenv.cabal
+++ b/dotenv.cabal
@@ -1,5 +1,5 @@
 name:                dotenv
-version:             0.10.0.1
+version:             0.10.1.0
 synopsis:            Loads environment variables from dotenv files
 homepage:            https://github.com/stackbuilders/dotenv-hs
 description:
@@ -96,7 +96,7 @@
                        , shellwords >= 0.1.3.0
                        , text
                        , exceptions >= 0.8 && < 0.11
-                       , mtl >= 2.3 && < 2.4
+                       , mtl >= 2.2.2 && < 2.4
 
   hs-source-dirs:      src
   ghc-options:         -Wall
diff --git a/spec/Configuration/DotenvSpec.hs b/spec/Configuration/DotenvSpec.hs
--- a/spec/Configuration/DotenvSpec.hs
+++ b/spec/Configuration/DotenvSpec.hs
@@ -72,7 +72,7 @@
           unsetEnv "ANOTHER_ENV"
           loadFile config `shouldThrow` anyErrorCall
 
-      context "when the needed env vars are not missing" $
+      context "when the needed env vars are not missing" $ do
         it "should succeed when loading all of the needed env vars" $ do
           -- Load extra information
           me <- init <$> readCreateProcess (shell "whoami") ""
@@ -87,6 +87,13 @@
           lookupEnv "DOTENV" `shouldReturn` Just "true"
           lookupEnv "UNICODE_TEST" `shouldReturn` Just "Manabí"
           lookupEnv "ANOTHER_ENV" `shouldReturn` Just "hello"
+
+        it "succeeds when variable is preset" $ do
+          setEnv "DOTENV" "preset"
+
+          loadFile config
+
+          lookupEnv "DOTENV" `shouldReturn` Just "preset"
 
   describe "parseFile" $ after_ clearEnvs $ do
     it "returns variables from a file without changing the environment" $ do
diff --git a/src/Configuration/Dotenv.hs b/src/Configuration/Dotenv.hs
--- a/src/Configuration/Dotenv.hs
+++ b/src/Configuration/Dotenv.hs
@@ -31,8 +31,10 @@
 import           Control.Monad                       (unless, when)
 import           Control.Monad.Catch
 import           Control.Monad.IO.Class              (MonadIO (..))
-import           Data.List                           (intersectBy, union,
-                                                      unionBy)
+import           Data.Function                       (on)
+import           Data.List.NonEmpty                  (NonEmpty(..))
+import qualified Data.List.NonEmpty as NE
+import           Data.List                           ((\\), intercalate, union)
 import           System.IO.Error                     (isDoesNotExistError)
 import           Text.Megaparsec                     (errorBundlePretty, parse)
 
@@ -59,22 +61,36 @@
   -> m ()
 loadFile config@Config {..} = do
   environment <- liftIO getEnvironment
-  readVars <- fmap concat (mapM parseFile configPath)
-  neededVars <- fmap concat (mapM parseFile configExamplePath)
-  let coincidences = (environment `union` readVars) `intersectEnvs` neededVars
-      cmpEnvs env1 env2 = fst env1 == fst env2
-      intersectEnvs = intersectBy cmpEnvs
-      unionEnvs = unionBy cmpEnvs
-      vars =
-        if (not . null) neededVars
-          then if length neededVars == length coincidences
-                 then readVars `unionEnvs` neededVars
-                 else error $
-                      "Missing env vars! Please, check (this/these) var(s) (is/are) set:" ++
-                      concatMap ((++) " " . fst) neededVars
-          else readVars
+
+  vars <- case (NE.nonEmpty configPath, NE.nonEmpty configExamplePath) of
+    (Nothing, _) -> pure []
+    (Just envs, Nothing) -> concat <$> mapM parseFile envs
+    (Just envs, Just envExamples) -> do
+      readVars <- concat <$> mapM parseFile envs
+      neededKeys <- map fst . concat <$> mapM parseFile envExamples
+
+      let
+        presentKeys = (union `on` map fst) environment readVars
+        missingKeys = neededKeys \\ presentKeys
+
+      pure $
+        if null missingKeys
+          then readVars
+          else error $ concat
+            [ "The following variables are present in "
+            , showPaths "one of " envExamples
+            , ", but not set in the current environment, or "
+            , showPaths "any of " envs
+            , ": "
+            , intercalate ", " missingKeys
+            ]
+
   unless allowDuplicates $ (lookUpDuplicates . map fst) vars
   runReaderT (mapM_ applySetting vars) config
+ where
+  showPaths :: String -> NonEmpty FilePath -> String
+  showPaths _ (p:|[]) = p
+  showPaths prefix ps = prefix <> intercalate ", " (NE.toList ps)
 
 -- | Parses the given dotenv file and returns values /without/ adding them to
 -- the environment.