Consider the following file.
shader.txt

It contains a perfectly valid SPIR-V (i.e. the following test succeeds).

TEST(A, a) {
  std::string shader; // code from the file

  const auto env = SPV_ENV_UNIVERSAL_1_3;
  const auto consumer = nullptr;
  const auto context = BuildModule(env, consumer, shader, kFuzzAssembleOption);
  ASSERT_TRUE(IsValid(env, context.get()));
}

However, if we try to move a basic block with id 370 one block down, the transformation will be applied correctly (i.e. TransformationMoveBlockDown::IsApplicable returns true) but the spirv-val will fail with with a somewhat ambiguous message:

error: line 121: Case construct that targets 310[%310] has invalid branch to block 414[%414] (not another case construct, corresponding merge, outer loop merge or outer loop continue)
  %310 = OpLabel

Commit id is 7afbc0c.