diff --git a/src/proto_tcp.c b/src/proto_tcp.c
index 9778856..72dc92b 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -1048,8 +1048,8 @@
 				t = rule->act_prm.trk_ctr.table.t;
 				key = stktable_fetch_key(t, s->be, s, &s->txn, SMP_OPT_DIR_REQ | partial, rule->act_prm.trk_ctr.expr, &smp);
 
-				if (smp.flags & SMP_F_MAY_CHANGE)
-					goto missing_data;
+				if ((smp.flags & SMP_F_MAY_CHANGE) && !(partial & SMP_OPT_FINAL))
+					goto missing_data; /* key might appear later */
 
 				if (key && (ts = stktable_get_entry(t, key))) {
 					session_track_stkctr(&s->stkctr[tcp_trk_idx(rule->action)], t, ts);
diff --git a/src/sample.c b/src/sample.c
index 3a0f3fb..8e62640 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -896,6 +896,18 @@
  * Note: the fetch functions are required to properly set the return type. The
  * conversion functions must do so too. However the cast functions do not need
  * to since they're made to cast mutiple types according to what is required.
+ *
+ * The caller may indicate in <opt> if it considers the result final or not.
+ * The caller needs to check the SMP_F_MAY_CHANGE flag in p->flags to verify
+ * if the result is stable or not, according to the following table :
+ *
+ * return MAY_CHANGE FINAL   Meaning for the sample
+ *  NULL      0        *     Not present and will never be (eg: header)
+ *  NULL      1        0     Not present yet, could change (eg: POST param)
+ *  NULL      1        1     Not present yet, will not change anymore
+ *   smp      0        *     Present and will not change (eg: header)
+ *   smp      1        0     Present, may change (eg: request length)
+ *   smp      1        1     Present, last known value (eg: request length)
  */
 struct sample *sample_process(struct proxy *px, struct session *l4, void *l7,
                               unsigned int opt,
@@ -1153,7 +1165,16 @@
  * and <opt> does not contain SMP_OPT_FINAL, then the sample is returned as-is
  * with its SMP_F_MAY_CHANGE flag so that the caller can check it and decide to
  * take actions (eg: wait longer). If a sample could not be found or could not
- * be converted, NULL is returned.
+ * be converted, NULL is returned. The caller MUST NOT use the sample if the
+ * SMP_F_MAY_CHANGE flag is present, as it is used only as a hint that there is
+ * still hope to get it after waiting longer, and is not converted to string.
+ * The possible output combinations are the following :
+ *
+ * return MAY_CHANGE FINAL   Meaning for the sample
+ *  NULL      *        *     Not present and will never be (eg: header)
+ *   smp      0        *     Final value converted (eg: header)
+ *   smp      1        0     Not present yet, may appear later (eg: header)
+ *   smp      1        1     never happens (either flag is cleared on output)
  */
 struct sample *sample_fetch_string(struct proxy *px, struct session *l4, void *l7,
                                    unsigned int opt, struct sample_expr *expr)
diff --git a/src/stick_table.c b/src/stick_table.c
index a708d3c..d39b4ff 100644
--- a/src/stick_table.c
+++ b/src/stick_table.c
@@ -603,7 +603,16 @@
  * no key could be extracted, or a pointer to the converted result stored in
  * static_table_key in format <table_type>. If <smp> is not NULL, it will be reset
  * and its flags will be initialized so that the caller gets a copy of the input
- * sample, and knows why it was not accepted (eg: SMP_F_MAY_CHANGE is present).
+ * sample, and knows why it was not accepted (eg: SMP_F_MAY_CHANGE is present
+ * without SMP_OPT_FINAL). The output will be usable like this :
+ *
+ * return MAY_CHANGE FINAL   Meaning for the sample
+ *  NULL      0        *     Not present and will never be (eg: header)
+ *  NULL      1        0     Not present or unstable, could change (eg: req_len)
+ *  NULL      1        1     Not present, will not change anymore
+ *   smp      0        *     Present and will not change (eg: header)
+ *   smp      1        0     not possible
+ *   smp      1        1     Present, last known value (eg: request length)
  */
 struct stktable_key *stktable_fetch_key(struct stktable *t, struct proxy *px, struct session *l4, void *l7,
                                         unsigned int opt, struct sample_expr *expr, struct sample *smp)