diff --git a/js/compiler/compiler.js b/js/compiler/compiler.js
index 046d6f5..f674ace 100644
--- a/js/compiler/compiler.js
+++ b/js/compiler/compiler.js
@@ -152,7 +152,7 @@ var modifiedSourceActions = {
   },
 
   FacetSituation_assert: function(_assert, expr, whenClause, _sc) {
-    return '\n.addAssertion(' + buildSubscription([expr], 'assert', 'pattern', whenClause) + ')';
+    return '\n.addAssertion(' + buildSubscription([expr], 'assert', 'pattern', whenClause, null) + ')';
   },
   FacetSituation_event: function(_on, eventPattern, block) {
     return buildOnEvent(false,
@@ -166,18 +166,20 @@ var modifiedSourceActions = {
     return '\n.addOnEventHandler((function(' + id.asES5 + ') ' + block.asES5 + '))';
   },
   FacetSituation_during: function(_during, pattern, facetBlock) {
+    var cachedAssertionVar = gensym('cachedAssertion');
     return buildOnEvent(false,
                         'asserted',
                         pattern.subscription,
                         pattern.projection,
                         pattern.bindings,
                         '{ ' + facetBlock.facetVarDecls +
+                        '\nvar '+cachedAssertionVar+' = '+pattern.instantiatedAssertion+';'+
                         '\nSyndicate.Actor.createFacet()' +
                         facetBlock.asES5 +
                         buildOnEvent(true,
                                      'retracted',
-                                     pattern.instantiatedSubscription,
-                                     pattern.instantiatedProjection,
+                                     pattern.instantiatedSubscription(cachedAssertionVar),
+                                     pattern.instantiatedProjection(cachedAssertionVar),
                                      [],
                                      '{}') +
                         '.completeBuild(); }');
@@ -221,7 +223,7 @@ semantics.addAttribute('eventType', {
   FacetTransitionEventPattern_risingEdge: function (_lp, expr, _rp) { return 'risingEdge'; }
 });
 
-function buildSubscription(children, patchMethod, mode, whenClause) {
+function buildSubscription(children, patchMethod, mode, whenClause, cachedAssertionVar) {
   var fragments = [];
   var hasWhenClause = (whenClause && (whenClause.numChildren === 1));
   fragments.push('(function() { var _ = Syndicate.__; return ');
@@ -233,7 +235,11 @@ function buildSubscription(children, patchMethod, mode, whenClause) {
   } else {
     fragments.push('{ assertion: ');
   }
-  children.forEach(function (c) { c.buildSubscription(fragments, mode); });
+  if (cachedAssertionVar) {
+    fragments.push(cachedAssertionVar);
+  } else {
+    children.forEach(function (c) { c.buildSubscription(fragments, mode); });
+  }
   if (patchMethod) {
     fragments.push(', ');
   } else {
@@ -254,25 +260,35 @@ function buildSubscription(children, patchMethod, mode, whenClause) {
 
 semantics.addAttribute('subscription', {
   _default: function(children) {
-    return buildSubscription(children, 'sub', 'pattern', null);
+    return buildSubscription(children, 'sub', 'pattern', null, null);
   }
 });
 
-semantics.addAttribute('instantiatedSubscription', {
+semantics.addAttribute('instantiatedAssertion', {
   _default: function(children) {
-    return buildSubscription(children, 'sub', 'instantiated', null);
+    var fragments = [];
+    fragments.push('(function() { var _ = Syndicate.__; return ');
+    children.forEach(function (c) { c.buildSubscription(fragments, 'instantiated'); });
+    fragments.push('; })()');
+    return fragments.join('');
   }
 });
 
-semantics.addAttribute('instantiatedProjection', {
+semantics.addOperation('instantiatedSubscription(cachedAssertionVar)', {
   _default: function(children) {
-    return buildSubscription(children, null, 'instantiated', null);
+    return buildSubscription(children, 'sub', 'instantiated', null, this.args.cachedAssertionVar);
+  }
+});
+
+semantics.addOperation('instantiatedProjection(cachedAssertionVar)', {
+  _default: function(children) {
+    return buildSubscription(children, null, 'instantiated', null, this.args.cachedAssertionVar);
   }
 });
 
 semantics.addAttribute('projection', {
   _default: function(children) {
-    return buildSubscription(children, null, 'projection', null);
+    return buildSubscription(children, null, 'projection', null, null);
   }
 });
 
diff --git a/js/compiler/demo-during-criterion-snapshotting.js b/js/compiler/demo-during-criterion-snapshotting.js
new file mode 100644
index 0000000..032f9af
--- /dev/null
+++ b/js/compiler/demo-during-criterion-snapshotting.js
@@ -0,0 +1,40 @@
+// Illustrates a (now fixed) bug where mutation altering a
+// subscription caused the `retracted` half of a during instance to be
+// lost.
+//
+// Symptomatic output:
+// x= 123 v= 999
+// x= 124 v= 999
+//
+// Correct output:
+// x= 123 v= 999
+// finally for x= 124 v= 999
+// x= 124 v= 999
+//
+// Should eventually be turned into some kind of test case.
+
+var Syndicate = require('./src/main.js');
+var Dataspace = Syndicate.Dataspace;
+
+assertion type foo(x, y);
+
+ground dataspace {
+  actor {
+    var x = 123;
+    react {
+      assert foo(x, 999);
+
+      during foo(x, $v) {
+        do {
+          console.log('x=', x, 'v=', v);
+          if (x === 123) {
+            x = 124;
+          }
+        }
+        finally {
+          console.log('finally for x=', x, 'v=', v);
+        }
+      }
+    }
+  }
+}
diff --git a/racket/syndicate/actor.rkt b/racket/syndicate/actor.rkt
index 9ed32f0..ec58243 100644
--- a/racket/syndicate/actor.rkt
+++ b/racket/syndicate/actor.rkt
@@ -624,7 +624,7 @@
     (define (analyze-during! index P-stx O-stxs)
       (define E-stx #`(asserted #,P-stx))
       (define-values (_proj _pat _bindings instantiated) (analyze-pattern E-stx P-stx))
-      (define I-stx #`(until (retracted #,instantiated) #,@O-stxs))
+      (define I-stx #`(let ((p #,instantiated)) (until (retracted p) #,@O-stxs)))
       (analyze-event! index E-stx #`(#,I-stx)))
 
     (define (analyze-assertion! index Pred-stx outer-expr-stx P-stx L-stx)
diff --git a/racket/syndicate/examples/actor/example-during-criterion-snapshotting.rkt b/racket/syndicate/examples/actor/example-during-criterion-snapshotting.rkt
new file mode 100644
index 0000000..262f4a4
--- /dev/null
+++ b/racket/syndicate/examples/actor/example-during-criterion-snapshotting.rkt
@@ -0,0 +1,27 @@
+#lang syndicate
+;; Illustrates a (now fixed) bug where mutation altering a
+;; subscription caused the `retracted` half of a during instance to be
+;; lost.
+;;
+;; Symptomatic output:
+;; x=123 v=999
+;; x=124 v=999
+;;
+;; Correct output:
+;; x=123 v=999
+;; finally for x=124 v=999
+;; x=124 v=999
+;;
+;; Should eventually be turned into some kind of test case.
+
+(require syndicate/actor)
+
+(struct foo (x y) #:prefab)
+
+(actor (define x 123)
+       (forever
+        (assert (foo x 999))
+        (during (foo x $v)
+                #:init [(log-info "x=~a v=~a" x v)
+                        (when (= x 123) (set! x 124))]
+                #:done [(log-info "finally for x=~a v=~a" x v)])))