diff --git a/spike/x86/survey.sh b/spike/x86/survey.sh
index 8a245f5..8c0678e 100755
--- a/spike/x86/survey.sh
+++ b/spike/x86/survey.sh
@@ -92,6 +92,26 @@ forever="dev-loop dev-watch dev-chatty agent-auto"
 # in the epilogue that every exit already went through. The five are in the
 # sweep now and they are five of the MATCHes.
 
+# The ones whose whole point is a fault, and which therefore cannot be compared
+# at this sweep's optimisation level. Both write through a bytes-view of a
+# string literal, which is a store into .rodata: measured here, LLVM exits 0
+# having printed the unmodified literal and this backend exits 139, because the
+# store is undefined and the optimiser deleted it on one side and there is no
+# optimiser on the other. That is not a lowering disagreement. At -O0 the two
+# agree exactly -- 139, no output, both backends -- and test_acceptance.ml's
+# dies_segv rows pin precisely that, on both backends, which is the coverage
+# this sweep would otherwise be duplicating at the one level where, as that
+# file's own comment puts it, there is nothing left to pin but the UB.
+#
+# Excluded by name rather than by building the whole corpus at -O0: the counts
+# below are one measurement and every handoff's baseline was taken against it.
+#
+# dev-segv would not belong here even if the store survived. It calls
+# agent/start, so it leaves a socket in /tmp on both runs, and under
+# SURVEY_FLAGS=--dev it parks in the break loop instead of dying -- two
+# truncations at the timeout, which is the dev-chatty failure by another road.
+faults="bytes-view-write dev-segv"
+
 TIMEOUT=${TIMEOUT:-20}
 
 # Extra flags, given to *both* sides. SURVEY_FLAGS=--dev is the one that has a
@@ -115,6 +135,7 @@ for src in "$corpus"/test/programs/*.flan "$corpus"/spike/x86/*.flan \
     [ $want = 1 ] || continue
   fi
   case " $forever " in *" $name "*) skip+=("$name:runs-forever"); continue;; esac
+  case " $faults " in *" $name "*) skip+=("$name:faults-by-design"); continue;; esac
 
   # LLVM first. A program that does not compile at all, or has no main, is not
   # this backend's business -- the frontend refused it either way.