diff --git a/src/testdir/test_signals.vim b/src/testdir/test_signals.vim
index 135037a..78b2cec 100644
--- a/src/testdir/test_signals.vim
+++ b/src/testdir/test_signals.vim
@@ -123,9 +123,15 @@
   if cmd =~ 'valgrind'
     throw 'Skipped: cannot test signal TERM with valgrind'
   endif
+
+  " If test fails once, it can leave temporary files and trying to rerun
+  " the test would then fail again if they are not deleted first.
+  call delete('.Xsig_TERM.swp')
+  call delete('XsetupAucmd')
+  call delete('XautoOut')
   let lines =<< trim END
-    au VimLeave * call writefile(["VimLeave triggered"], "XautoOut", "a")
-    au VimLeavePre * call writefile(["VimLeavePre triggered"], "XautoOut", "a")
+    au VimLeave * call writefile(["VimLeave triggered"], "XautoOut", "as")
+    au VimLeavePre * call writefile(["VimLeavePre triggered"], "XautoOut", "as")
   END
   call writefile(lines, 'XsetupAucmd')