diff --git a/test/test_valgrind.ml b/test/test_valgrind.ml
index d158ae7..0ee7b8c 100644
--- a/test/test_valgrind.ml
+++ b/test/test_valgrind.ml
@@ -122,12 +122,38 @@ let summary text =
 
 let suppressed_total = ref 0
 
+(* Files a corpus program leaves in the working directory, cleared before
+   *every* run of it.
+
+   Not housekeeping: two of these programs are not idempotent, and running one
+   twice — which is precisely what this harness does, once plain and once
+   under memcheck — makes the second run print something different from the
+   first. slurp.flan is the case that found it. Its last section sets up a
+   missing file, expects the handler to fire, and has the handler barf the
+   file into existence and invoke retry; on a second run the file is already
+   there, slurp succeeds first time, and the handler count prints 0 where the
+   first run printed 1. That looks exactly like "the program behaves
+   differently under memcheck" and is nothing of the kind — measured by
+   running it twice with no valgrind anywhere near it, which reproduces the 0.
+
+   test_acceptance.ml removes the same two names for the same reason; this is
+   that list, not a new fact about the corpus. It matters here and not in
+   test_sanitize.ml only because slurp.flan is in this sweep's corpus and not
+   in that one's. *)
+let artifacts =
+  [ "slurp-out.txt"; "slurp-made.txt"; "web-files-out.txt" ]
+
+let clean () =
+  List.iter (fun f -> try Sys.remove f with Sys_error _ -> ()) artifacts
+
 (* One program, both ways. *)
 let check label path args ~checks =
   match compile ~checks path with
   | exception Failure m -> fail "%s %s: build: %s" label path m
   | exe ->
+    clean ();
     let c1, t1, _ = run exe args in
+    clean ();
     let c2, t2, log = run ~vg:true exe args in
     (match summary log with
      | None ->
@@ -140,6 +166,7 @@ let check label path args ~checks =
       fail "%s %s: diverged under memcheck\n  \
             plain (exit %d): %S\n  memcheck (exit %d): %S"
         label path c1 t1 c2 t2;
+    clean ();
     (try Sys.remove exe with Sys_error _ -> ())
 
 (* The corpus. This list is *larger* than @sanitize's, deliberately: that one