diff --git a/CHANGELOG.yaml b/CHANGELOG.yaml
--- a/CHANGELOG.yaml
+++ b/CHANGELOG.yaml
@@ -1,6 +1,13 @@
 upcoming:
 
 releases:
+  - version: "0.0.4.1"
+    changes:
+      - description: Backport "should not happen" error fixes and configurable manager
+        issue: 47, 48
+        authors: parsonsmatt
+        date: 2018-05-10
+
   - version: "0.0.4"
     changes:
 
diff --git a/servant-quickcheck.cabal b/servant-quickcheck.cabal
--- a/servant-quickcheck.cabal
+++ b/servant-quickcheck.cabal
@@ -1,5 +1,5 @@
 name:                servant-quickcheck
-version:             0.0.4
+version:             0.0.4.1
 synopsis:            QuickCheck entire APIs
 description:
  This packages provides QuickCheck properties that are tested across an entire
diff --git a/src/Servant/QuickCheck/Internal/QuickCheck.hs b/src/Servant/QuickCheck/Internal/QuickCheck.hs
--- a/src/Servant/QuickCheck/Internal/QuickCheck.hs
+++ b/src/Servant/QuickCheck/Internal/QuickCheck.hs
@@ -2,9 +2,10 @@
 {-# LANGUAGE CPP #-}
 module Servant.QuickCheck.Internal.QuickCheck where
 
-import           Control.Concurrent       (modifyMVar_, newMVar, readMVar)
+import           Control.Concurrent       (tryReadMVar, newEmptyMVar, tryPutMVar)
 import           Control.Monad            (unless)
 import qualified Data.ByteString.Lazy     as LBS
+import           Data.Monoid              ((<>))
 import           Data.Proxy               (Proxy)
 import qualified Network.HTTP.Client      as C
 import           Network.Wai.Handler.Warp (withApplication)
@@ -73,18 +74,23 @@
   let reqs = (\f -> (f burl1, f burl2)) <$> runGenRequest api
   -- This MVar stuff is clunky! But there doesn't seem to be an easy way to
   -- return results when a test fails, since an exception is throw.
-  deetsMVar <- newMVar $ error "should not be called"
+  deetsMVar <- newEmptyMVar
   r <- quickCheckWithResult args { chatty = False } $ monadicIO $ forAllM reqs $ \(req1, req2) -> do
     resp1 <- run $ C.httpLbs (noCheckStatus req1) defManager
     resp2 <- run $ C.httpLbs (noCheckStatus req2) defManager
     unless (getResponseEquality req resp1 resp2) $ do
       monitor (counterexample "hi" )
-      run $ modifyMVar_ deetsMVar $ const $ return $
-        ServerEqualityFailure req1 resp1 resp2
+      _ <- run $ tryPutMVar deetsMVar $ ServerEqualityFailure req1 resp1 resp2
       assert False
   case r of
     Success {} -> return ()
-    Failure{..} -> readMVar deetsMVar >>= \x -> expectationFailure $ "Failed:\n" ++ show x
+    Failure{..} -> do
+      mx <- tryReadMVar deetsMVar
+      case mx of
+        Just x ->
+          expectationFailure $ "Failed:\n" ++ show x
+        Nothing ->
+          expectationFailure $ "We failed to record a reason for failure: " <> show r
     GaveUp { numTests = n } -> expectationFailure $ "Gave up after " ++ show n ++ " tests"
     NoExpectedFailure {} -> expectationFailure "No expected failure"
     InsufficientCoverage {} -> expectationFailure "Insufficient coverage"
@@ -110,30 +116,40 @@
 -- /Since 0.0.0.0/
 serverSatisfies :: (HasGenRequest a) =>
   Proxy a -> BaseUrl -> Args -> Predicates -> Expectation
-serverSatisfies api burl args preds = do
+serverSatisfies api =  serverSatisfiesMgr api defManager
+
+serverSatisfiesMgr :: (HasGenRequest a) =>
+  Proxy a -> C.Manager -> BaseUrl -> Args -> Predicates -> Expectation
+serverSatisfiesMgr api manager burl args preds = do
   let reqs = ($ burl) <$> runGenRequest api
-  deetsMVar <- newMVar $ error "should not be called"
+  deetsMVar <- newEmptyMVar
   r <- quickCheckWithResult args { chatty = False } $ monadicIO $ forAllM reqs $ \req -> do
-     v <- run $ finishPredicates preds (noCheckStatus req) defManager
-     run $ modifyMVar_ deetsMVar $ const $ return v
+     v <- run $ finishPredicates preds (noCheckStatus req) manager
+     _ <- run $ tryPutMVar deetsMVar v
      case v of
        Just _ -> assert False
        _ -> return ()
   case r of
     Success {} -> return ()
-    Failure{..} -> readMVar deetsMVar >>= \x -> expectationFailure $
-      "Failed:\n" ++ show x
+    Failure {..} -> do
+      mx <- tryReadMVar deetsMVar
+      case mx of
+        Just x -> expectationFailure $ "Failed:\n" ++ show x
+        Nothing -> expectationFailure $ "Failed to retrieve error. QC Failure: " <> show r
     GaveUp { numTests = n } -> expectationFailure $ "Gave up after " ++ show n ++ " tests"
     NoExpectedFailure {} -> expectationFailure $ "No expected failure"
     InsufficientCoverage {} -> expectationFailure $ "Insufficient coverage"
 
-
 serverDoesntSatisfy :: (HasGenRequest a) =>
   Proxy a -> BaseUrl -> Args -> Predicates -> Expectation
-serverDoesntSatisfy api burl args preds = do
+serverDoesntSatisfy api = serverDoesntSatisfyMgr api defManager
+
+serverDoesntSatisfyMgr :: (HasGenRequest a) =>
+  Proxy a -> C.Manager -> BaseUrl -> Args -> Predicates -> Expectation
+serverDoesntSatisfyMgr api manager burl args preds = do
   let reqs = ($ burl) <$> runGenRequest api
   r <- quickCheckWithResult args $ monadicIO $ forAllM reqs $ \req -> do
-     v <- run $ finishPredicates preds (noCheckStatus req) defManager
+     v <- run $ finishPredicates preds (noCheckStatus req) manager
      assert $ not $ null v
   case r of
     Success {} -> return ()