diff --git a/unit-tests/where-does-it-break.sh b/unit-tests/where-does-it-break.sh
new file mode 100755
index 0000000..5c51aca
--- /dev/null
+++ b/unit-tests/where-does-it-break.sh
@@ -0,0 +1,29 @@
+#!/bin/bash
+
+# Not really a unit test, but a check to see where bignum addition breaks
+
+broken=0
+i=1152921506900200000
+# we've already proven we can successfullu get up to here
+increment=10000
+
+while [ $broken -eq "0" ]
+do
+  expr="(+ $i $increment)"
+  # Use sbcl as our reference implementation...
+  expected=`echo "$expr" | sbcl --noinform | grep -v '*'`
+  actual=`echo "$expr" | target/psse | tail -1 | sed 's/\,//g'`
+
+  echo -n "adding $increment to $i: "
+
+  if [ "${expected}" = "${actual}" ]
+  then
+      echo "OK"
+  else
+      echo "Fail: expected '${expected}', got '${actual}'"
+      broken=1
+      exit 1
+  fi
+
+  i=$expected
+done