diff --git a/Yesod/Core/Internal/Run.hs b/Yesod/Core/Internal/Run.hs
--- a/Yesod/Core/Internal/Run.hs
+++ b/Yesod/Core/Internal/Run.hs
@@ -13,6 +13,7 @@
 import           Control.Exception            (fromException, bracketOnError, evaluate)
 import qualified Control.Exception            as E
 import           Control.Exception.Lifted     (catch)
+import           Control.Monad                (mplus)
 import           Control.Monad.IO.Class       (MonadIO)
 import           Control.Monad.IO.Class       (liftIO)
 import           Control.Monad.Logger         (LogLevel (LevelError), LogSource,
@@ -48,6 +49,7 @@
 import           Yesod.Core.Internal.Request  (parseWaiRequest,
                                                tooLargeResponse)
 import           Yesod.Routes.Class           (Route, renderRoute)
+import Control.DeepSeq (($!!))
 
 -- | Function used internally by Yesod in the process of converting a
 -- 'HandlerT' into an 'Application'. Should not be needed by users.
@@ -78,23 +80,35 @@
         (\e -> return $ Left $ maybe (HCError $ toErrorHandler e) id
                       $ fromException e)
     state <- liftIO $ I.readIORef istate
-    let finalSession = ghsSession state
-    let headers = ghsHeaders state
-    let contents = either id (HCContent defaultStatus . toTypedContent) contents'
+
+    (finalSession, mcontents1) <- (do
+        finalSession <- return $!! ghsSession state
+        return (finalSession, Nothing)) `E.catch` \e -> return
+            (Map.empty, Just $! HCError $! InternalError $! T.pack $! show (e :: E.SomeException))
+
+    (headers, mcontents2) <- (do
+        headers <- return $!! appEndo (ghsHeaders state) []
+        return (headers, Nothing)) `E.catch` \e -> return
+            ([], Just $! HCError $! InternalError $! T.pack $! show (e :: E.SomeException))
+
+    let contents =
+            case mcontents1 `mplus` mcontents2 of
+                Just x -> x
+                Nothing -> either id (HCContent defaultStatus . toTypedContent) contents'
     let handleError e = flip runInternalState resState $ do
             yar <- rheOnError e yreq
                 { reqSession = finalSession
                 }
             case yar of
                 YRPlain status' hs ct c sess ->
-                    let hs' = appEndo headers hs
+                    let hs' = headers ++ hs
                         status
                             | status' == defaultStatus = getStatus e
                             | otherwise = status'
                      in return $ YRPlain status hs' ct c sess
                 YRWai _ -> return yar
     let sendFile' ct fp p =
-            return $ YRPlain H.status200 (appEndo headers []) ct (ContentFile fp p) finalSession
+            return $ YRPlain H.status200 headers ct (ContentFile fp p) finalSession
     contents1 <- evaluate contents `E.catch` \e -> return
         (HCError $! InternalError $! T.pack $! show (e :: E.SomeException))
     case contents1 of
@@ -102,7 +116,7 @@
             ec' <- liftIO $ evaluateContent c
             case ec' of
                 Left e -> handleError e
-                Right c' -> return $ YRPlain status (appEndo headers []) ct c' finalSession
+                Right c' -> return $ YRPlain status headers ct c' finalSession
         HCError e -> handleError e
         HCRedirect status loc -> do
             let disable_caching x =
@@ -110,7 +124,7 @@
                     : Header "Expires" "Thu, 01 Jan 1970 05:05:05 GMT"
                     : x
                 hs = (if status /= H.movedPermanently301 then disable_caching else id)
-                      $ Header "Location" (encodeUtf8 loc) : appEndo headers []
+                      $ Header "Location" (encodeUtf8 loc) : headers
             return $ YRPlain
                 status hs typePlain emptyContent
                 finalSession
@@ -118,7 +132,7 @@
             (sendFile' ct fp p)
             (handleError . toErrorHandler)
         HCCreated loc -> do
-            let hs = Header "Location" (encodeUtf8 loc) : appEndo headers []
+            let hs = Header "Location" (encodeUtf8 loc) : headers
             return $ YRPlain
                 H.status201
                 hs
diff --git a/Yesod/Core/Types.hs b/Yesod/Core/Types.hs
--- a/Yesod/Core/Types.hs
+++ b/Yesod/Core/Types.hs
@@ -64,6 +64,7 @@
 import           Yesod.Routes.Class                 (RenderRoute (..), ParseRoute (..))
 import           Control.Monad.Reader               (MonadReader (..))
 import Prelude hiding (catch)
+import Control.DeepSeq (NFData (rnf))
 
 -- Sessions
 type SessionMap = Map Text ByteString
@@ -312,6 +313,14 @@
     | Header ByteString ByteString
     deriving (Eq, Show)
 
+-- FIXME In the next major version bump, let's just add strictness annotations
+-- to Header (and probably everywhere else). We can also add strictness
+-- annotations to SetCookie in the cookie package.
+instance NFData Header where
+    rnf (AddCookie x) = rnf x
+    rnf (DeleteCookie x y) = rnf x `seq` rnf y
+    rnf (Header x y) = rnf x `seq` rnf y
+
 data Location url = Local url | Remote Text
     deriving (Show, Eq)
 
@@ -415,6 +424,13 @@
   uninterruptibleMask a =
     HandlerT $ \e -> uninterruptibleMask $ \u -> unHandlerT (a $ q u) e
       where q u (HandlerT b) = HandlerT (u . b)
+instance MonadCatch m => MonadCatch (WidgetT site m) where
+  catch (WidgetT m) c = WidgetT $ \r -> m r `catch` \e -> unWidgetT (c e) r
+  mask a = WidgetT $ \e -> mask $ \u -> unWidgetT (a $ q u) e
+    where q u (WidgetT b) = WidgetT (u . b)
+  uninterruptibleMask a =
+    WidgetT $ \e -> uninterruptibleMask $ \u -> unWidgetT (a $ q u) e
+      where q u (WidgetT b) = WidgetT (u . b)
 #else
     monadThrow = lift . monadThrow
 #endif
diff --git a/test/YesodCoreTest/ErrorHandling.hs b/test/YesodCoreTest/ErrorHandling.hs
--- a/test/YesodCoreTest/ErrorHandling.hs
+++ b/test/YesodCoreTest/ErrorHandling.hs
@@ -15,6 +15,9 @@
 import Network.HTTP.Types (mkStatus)
 import Blaze.ByteString.Builder (Builder, fromByteString, toLazyByteString)
 import Data.Monoid (mconcat)
+import Data.Text (Text, pack)
+import Control.Monad (forM_)
+import qualified Control.Exception.Lifted as E
 
 data App = App
 
@@ -26,6 +29,7 @@
 /error-in-body ErrorInBodyR GET
 /error-in-body-noeval ErrorInBodyNoEvalR GET
 /override-status OverrideStatusR GET
+/error/#Int ErrorR GET
 
 -- https://github.com/yesodweb/yesod/issues/658
 /builder BuilderR GET
@@ -98,6 +102,18 @@
 getGoodBuilderR :: Handler TypedContent
 getGoodBuilderR = return $ TypedContent "text/plain" $ toContent goodBuilderContent
 
+getErrorR :: Int -> Handler ()
+getErrorR 1 = setSession undefined "foo"
+getErrorR 2 = setSession "foo" undefined
+getErrorR 3 = deleteSession undefined
+getErrorR 4 = addHeader undefined "foo"
+getErrorR 5 = addHeader "foo" undefined
+getErrorR 6 = expiresAt undefined
+getErrorR 7 = setLanguage undefined
+getErrorR 8 = cacheSeconds undefined
+getErrorR 9 = setUltDest (undefined :: Text)
+getErrorR 10 = setMessage undefined
+
 errorHandlingTest :: Spec
 errorHandlingTest = describe "Test.ErrorHandling" $ do
       it "says not found" caseNotFound
@@ -110,6 +126,7 @@
       it "file with bad len" caseFileBadLen
       it "file with bad name" caseFileBadName
       it "builder includes content-length" caseGoodBuilder
+      forM_ [1..10] $ \i -> it ("error case " ++ show i) (caseError i)
 
 runner :: Session () -> IO ()
 runner f = toWaiApp App >>= runSession f
@@ -194,3 +211,10 @@
     let lbs = toLazyByteString goodBuilderContent
     assertBody lbs res
     assertHeader "content-length" (S8.pack $ show $ L.length lbs) res
+
+caseError :: Int -> IO ()
+caseError i = runner $ do
+    res <- request defaultRequest { pathInfo = ["error", pack $ show i] }
+    assertStatus 500 res `E.catch` \e -> do
+        liftIO $ print res
+        E.throwIO (e :: E.SomeException)
diff --git a/yesod-core.cabal b/yesod-core.cabal
--- a/yesod-core.cabal
+++ b/yesod-core.cabal
@@ -1,5 +1,5 @@
 name:            yesod-core
-version:         1.2.12
+version:         1.2.13
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>
@@ -47,7 +47,7 @@
                    , containers            >= 0.2
                    , monad-control         >= 0.3      && < 0.4
                    , transformers-base     >= 0.4
-                   , cookie                >= 0.4      && < 0.5
+                   , cookie                >= 0.4.1    && < 0.5
                    , http-types            >= 0.7
                    , case-insensitive      >= 0.2
                    , parsec                >= 2        && < 3.2
@@ -69,6 +69,7 @@
                    , unix-compat
                    , conduit-extra
                    , exceptions
+                   , deepseq
 
     exposed-modules: Yesod.Core
                      Yesod.Core.Content