diff --git a/CMakeLists.txt b/CMakeLists.txt
index e6feafc3..61edc7b6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -283,7 +283,7 @@ if(BUILD_EXTERNAL AND IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/External)
     add_subdirectory(External)
 endif()
 
-option(ALLOW_EXTERNAL_SPIRV_TOOLS "Allows to build against installed SPIRV-Tools-opt")
+option(ALLOW_EXTERNAL_SPIRV_TOOLS "Allows to build against installed SPIRV-Tools-opt. This is unsupported if the commit isn't the one in known_good.json")
 if(NOT TARGET SPIRV-Tools-opt)
     if(ALLOW_EXTERNAL_SPIRV_TOOLS)
         # Look for external SPIR-V Tools build, if not building in-tree
diff --git a/README.md b/README.md
index 44388108..5493ff90 100644
--- a/README.md
+++ b/README.md
@@ -264,6 +264,9 @@ Test](gtests/), one is the [`runtests` script](Test/runtests). The former
 runs unit tests and single-shader single-threaded integration tests, while
 the latter runs multiple-shader linking tests and multi-threaded tests.
 
+Tests may erroneously fail or pass if using `ALLOW_EXTERNAL_SPIRV_TOOLS` with
+any commit other than the one specified in `known_good.json`.
+
 ### Running tests
 
 The [`runtests` script](Test/runtests) requires compiled binaries to be