diff --git a/README.coding.md b/README.coding.md
index 1e3f171f..a4da4db7 100644
--- a/README.coding.md
+++ b/README.coding.md
@@ -36,6 +36,19 @@ with the socket closing and the `wsi` freed.
 Websocket write activities should only take place in the
 `LWS_CALLBACK_SERVER_WRITEABLE` callback as described below.
 
+[This network-programming necessity to link the issue of new data to
+the peer taking the previous data is not obvious to all users so let's
+repeat that in other words:
+
+***ONLY DO LWS_WRITE FROM THE WRITEABLE CALLBACK***
+
+There is another network-programming truism that surprises some people which
+is if the sink for the data cannot accept more:
+
+***YOU MUST PERFORM RX FLOW CONTROL***
+
+See the mirror protocol implementations for example code.
+
 Only live connections appear in the user callbacks, so this removes any
 possibility of trying to used closed and freed wsis.
 
diff --git a/lib/output.c b/lib/output.c
index 3398fd13..f14a4a3f 100644
--- a/lib/output.c
+++ b/lib/output.c
@@ -109,7 +109,9 @@ int lws_issue_raw(struct lws *wsi, unsigned char *buf, size_t len)
 	if (wsi->trunc_len && (buf < wsi->trunc_alloc ||
 	    buf > (wsi->trunc_alloc + wsi->trunc_len +
 		   wsi->trunc_offset))) {
-		lwsl_err("****** %x Sending new, pending truncated ...\n", wsi);
+		lwsl_err("****** %p: Sending new, pending truncated ...\n"
+			 "       It's illegal to do an lws_write outside of\n"
+			 "       the writable callback: fix your code", wsi);
 		assert(0);
 
 		return -1;