diff --git a/collects/tests/mzscheme/thread.ss b/collects/tests/mzscheme/thread.ss
index 0929a6fb1a..7cfcb2cf69 100644
--- a/collects/tests/mzscheme/thread.ss
+++ b/collects/tests/mzscheme/thread.ss
@@ -1275,4 +1275,13 @@
 
 ; --------------------
 
+;; Make sure shared thread cell is not exposed:
+(test #f 'no-breaks (with-handlers ([void (lambda (x) (break-enabled))]) (/ 0)))
+(test #t 'no-breaks (with-handlers ([void (lambda (x) (break-enabled #t) (break-enabled))]) (/ 0)))
+(test #f 'no-breaks (with-handlers ([void (lambda (x) (break-enabled))]) (/ 0)))
+(test #t 'no-breaks (with-handlers ([(lambda (x) (break-enabled #t)) (lambda (x) (break-enabled))]) (/ 0)))
+(test #f 'no-breaks (with-handlers ([void (lambda (x) (break-enabled))]) (/ 0)))
+
+; --------------------
+
 (report-errs)
diff --git a/collects/tests/mzscheme/will.ss b/collects/tests/mzscheme/will.ss
index a047468886..766043ef2b 100644
--- a/collects/tests/mzscheme/will.ss
+++ b/collects/tests/mzscheme/will.ss
@@ -125,6 +125,14 @@
   (custodian-shutdown-all c1)
   (test '(#f #f) map custodian-box-value (list b1 b2)))
 
+(let ()
+  (let ([c (make-custodian)])
+    (let ([l (for/list ([i (in-range 32)])
+               (make-custodian-box c 7))])
+      (test #t andmap (lambda (b) (number? (custodian-box-value b))) l)
+      (custodian-shutdown-all c)
+      (test #f ormap (lambda (b) (number? (custodian-box-value b))) l))))
+
 ;; ----------------------------------------
 
 (report-errs)