diff --git a/lightningd/invoice.c b/lightningd/invoice.c
index 866f054f6..87ef0e31d 100644
--- a/lightningd/invoice.c
+++ b/lightningd/invoice.c
@@ -302,8 +302,15 @@ invoice_check_payment(const tal_t *ctx,
 	 *    - MUST fail the HTLC.
 	 *    - MUST return an `incorrect_or_unknown_payment_details` error.
 	 */
-	if (!wallet_invoice_find_unpaid(ld->wallet, &invoice, payment_hash))
+	if (!wallet_invoice_find_unpaid(ld->wallet, &invoice, payment_hash)) {
+		log_debug(ld->log, "Unknown paid invoice %s",
+			  type_to_string(tmpctx, struct sha256, payment_hash));
+		if (wallet_invoice_find_by_rhash(ld->wallet, &invoice, payment_hash)) {
+			log_debug(ld->log, "ALREADY paid invoice %s",
+				  type_to_string(tmpctx, struct sha256, payment_hash));
+		}
 		return NULL;
+	}
 
 	details = wallet_invoice_details(ctx, ld->wallet, invoice);
 
@@ -342,11 +349,22 @@ invoice_check_payment(const tal_t *ctx,
 	if (details->msat != NULL) {
 		struct amount_msat twice;
 
-		if (amount_msat_less(msat, *details->msat))
+		if (amount_msat_less(msat, *details->msat)) {
+			log_debug(ld->log, "Attept to pay %s with amount %s < %s",
+				  type_to_string(tmpctx, struct sha256,
+						 &details->rhash),
+				  type_to_string(tmpctx, struct amount_msat, &msat),
+				  type_to_string(tmpctx, struct amount_msat, details->msat));
 			return tal_free(details);
+		}
 
 		if (amount_msat_add(&twice, *details->msat, *details->msat)
 		    && amount_msat_greater(msat, twice)) {
+			log_debug(ld->log, "Attept to pay %s with amount %s > %s",
+				  type_to_string(tmpctx, struct sha256,
+						 &details->rhash),
+				  type_to_string(tmpctx, struct amount_msat, details->msat),
+				  type_to_string(tmpctx, struct amount_msat, &twice));
 			/* BOLT #4:
 			 *
 			 * - if the amount paid is more than twice the amount
diff --git a/lightningd/peer_htlcs.c b/lightningd/peer_htlcs.c
index 641a325b2..3d6a0eead 100644
--- a/lightningd/peer_htlcs.c
+++ b/lightningd/peer_htlcs.c
@@ -283,10 +283,13 @@ void local_fail_in_htlc_needs_update(struct htlc_in *hin,
 }
 
 /* Helper to create (common) WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS */
-const u8 *failmsg_incorrect_or_unknown(const tal_t *ctx,
-				       struct lightningd *ld,
-				       const struct htlc_in *hin)
+const u8 *failmsg_incorrect_or_unknown_(const tal_t *ctx,
+					struct lightningd *ld,
+					const struct htlc_in *hin,
+					const char *file, int line)
 {
+	log_debug(ld->log, "WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS: %s:%u",
+		  file, line);
 	return towire_incorrect_or_unknown_payment_details(
 		ctx, hin->msat,
 		get_block_height(ld->topology));
diff --git a/lightningd/peer_htlcs.h b/lightningd/peer_htlcs.h
index c24f76555..6d786c510 100644
--- a/lightningd/peer_htlcs.h
+++ b/lightningd/peer_htlcs.h
@@ -81,7 +81,11 @@ void json_format_forwarding_object(struct json_stream *response, const char *fie
 				   const struct forwarding *cur);
 
 /* Helper to create (common) WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS */
-const u8 *failmsg_incorrect_or_unknown(const tal_t *ctx,
-				       struct lightningd *ld,
-				       const struct htlc_in *hin);
+#define failmsg_incorrect_or_unknown(ctx, ld, hin) \
+	failmsg_incorrect_or_unknown_((ctx), (ld), (hin), __FILE__, __LINE__)
+
+const u8 *failmsg_incorrect_or_unknown_(const tal_t *ctx,
+					struct lightningd *ld,
+					const struct htlc_in *hin,
+					const char *file, int line);
 #endif /* LIGHTNING_LIGHTNINGD_PEER_HTLCS_H */
diff --git a/lightningd/test/run-invoice-select-inchan.c b/lightningd/test/run-invoice-select-inchan.c
index e7d426b0c..1d8e4ba9d 100644
--- a/lightningd/test/run-invoice-select-inchan.c
+++ b/lightningd/test/run-invoice-select-inchan.c
@@ -88,11 +88,12 @@ char *encode_scriptpubkey_to_addr(const tal_t *ctx UNNEEDED,
 				  const struct chainparams *chainparams UNNEEDED,
 				  const u8 *scriptPubkey UNNEEDED)
 { fprintf(stderr, "encode_scriptpubkey_to_addr called!\n"); abort(); }
-/* Generated stub for failmsg_incorrect_or_unknown */
-const u8 *failmsg_incorrect_or_unknown(const tal_t *ctx UNNEEDED,
-				       struct lightningd *ld UNNEEDED,
-				       const struct htlc_in *hin UNNEEDED)
-{ fprintf(stderr, "failmsg_incorrect_or_unknown called!\n"); abort(); }
+/* Generated stub for failmsg_incorrect_or_unknown_ */
+const u8 *failmsg_incorrect_or_unknown_(const tal_t *ctx UNNEEDED,
+					struct lightningd *ld UNNEEDED,
+					const struct htlc_in *hin UNNEEDED,
+					const char *file UNNEEDED, int line UNNEEDED)
+{ fprintf(stderr, "failmsg_incorrect_or_unknown_ called!\n"); abort(); }
 /* Generated stub for fatal */
 void   fatal(const char *fmt UNNEEDED, ...)
 { fprintf(stderr, "fatal called!\n"); abort(); }