I've found an issue with shouldNotTypeCheck where it's not capturing some of the deferred type errors at runtime.

$ cabal sandbox init
$ cabal install --dependencies-only --enable-tests
$ cabal test --show-details=always
...
Resolving dependencies...
Configuring shouldnottypecheck-bug-0.1.0.0...
Preprocessing test suite 'spec' for shouldnottypecheck-bug-0.1.0.0...
[1 of 2] Compiling Data.BugSpec     ( test/Data/BugSpec.hs, dist/build/spec/spec-tmp/Data/BugSpec.o )
[2 of 2] Compiling Main             ( test/Spec.hs, dist/build/spec/spec-tmp/Main.o )
Linking dist/build/spec/spec ...
Running 1 test suites...
Test suite spec: RUNNING...

Data.Bug
  shoultNotTypecheck
    captures errors from fromNatSingToZ
    captures errors from fromZToF . fromNatSingToZ FAILED [1]
    captures errors from fromZToF . (lte :=>)
    captures errors from fromNatSingToF FAILED [2]

Failures:

  src/Test/ShouldNotTypecheck.hs:40:
  1) Data.Bug.shoultNotTypecheck captures errors from fromZToF . fromNatSingToZ
       Make sure the expression has an NFData instance! See docs at https://github.com/CRogers/should-not-typecheck#nfdata-a-constraint. Full error:
       test/Data/BugSpec.hs:28:35: error:
           • Could not deduce (IsLessThanOrEqualTo ('Succ 'Zero) 'Zero)
               arising from a use of ‘fromNatSingToZ’
             from the context: () ~ ()
               bound by a type expected by the context:
                          () ~ () => F
               at test/Data/BugSpec.hs:28:5-70
           • In the first argument of ‘fromZToF’, namely
               ‘(fromNatSingToZ (SuccSing ZeroSing))’
             In the first argument of ‘shouldNotTypecheck’, namely
               ‘(fromZToF (fromNatSingToZ (SuccSing ZeroSing)))’
             In a stmt of a 'do' block:
               shouldNotTypecheck (fromZToF (fromNatSingToZ (SuccSing ZeroSing)))
       (deferred type error)

  src/Test/ShouldNotTypecheck.hs:40:
  2) Data.Bug.shoultNotTypecheck captures errors from fromNatSingToF
       Make sure the expression has an NFData instance! See docs at https://github.com/CRogers/should-not-typecheck#nfdata-a-constraint. Full error:
       test/Data/BugSpec.hs:36:25: error:
           • Could not deduce (IsLessThanOrEqualTo ('Succ 'Zero) 'Zero)
               arising from a use of ‘fromNatSingToF’
             from the context: () ~ ()
               bound by a type expected by the context:
                          () ~ () => F
               at test/Data/BugSpec.hs:36:5-59
           • In the first argument of ‘shouldNotTypecheck’, namely
               ‘(fromNatSingToF (SuccSing ZeroSing))’
             In a stmt of a 'do' block:
               shouldNotTypecheck (fromNatSingToF (SuccSing ZeroSing))
             In the second argument of ‘($)’, namely
               ‘do { shouldNotTypecheck (fromNatSingToF (SuccSing ZeroSing)) }’
       (deferred type error)

Randomized with seed 1234172950

Finished in 0.0058 seconds
4 examples, 2 failures

Test suite spec: FAIL
Test suite logged to: dist/test/shouldnottypecheck-bug-0.1.0.0-spec.log
0 of 1 test suites (0 of 1 test cases) passed.

Environment details

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.0.2
$ cabal --version
cabal-install version 1.24.0.2
compiled using version 1.24.2.0 of the Cabal library 
$ cabal sandbox hc-pkg list
/usr/local/Cellar/ghc/8.0.2/lib/ghc-8.0.2/package.conf.d
    Cabal-1.24.2.0
    array-0.5.1.1
    base-4.9.1.0
    binary-0.8.3.0
    bytestring-0.10.8.1
    containers-0.5.7.1
    deepseq-1.4.2.0
    directory-1.3.0.0
    filepath-1.4.1.1
    ghc-8.0.2
    ghc-boot-8.0.2
    ghc-boot-th-8.0.2
    ghc-prim-0.5.0.0
    ghci-8.0.2
    haskeline-0.7.3.0
    hoopl-3.10.2.1
    hpc-0.6.0.3
    integer-gmp-1.0.0.1
    pretty-1.1.3.3
    process-1.4.3.0
    rts-1.0
    template-haskell-2.11.1.0
    terminfo-0.4.0.2
    time-1.6.0.1
    transformers-0.5.2.0
    unix-2.7.2.1
    xhtml-3000.2.1
/Users/rampion/Projects/shouldnottypecheck-bug/.cabal-sandbox/x86_64-osx-ghc-8.0.2-packages.conf.d
    HUnit-1.6.0.0
    QuickCheck-2.9.2
    ansi-terminal-0.6.2.3
    async-2.1.1.1
    call-stack-0.1.0
    hspec-2.4.3
    hspec-core-2.4.3
    hspec-discover-2.4.3
    hspec-expectations-0.8.2
    primitive-0.6.2.0
    quickcheck-io-0.1.4
    random-1.1
    setenv-0.1.1.3
    should-not-typecheck-2.1.0
    stm-2.4.4.1
    tf-random-0.5