diff --git a/doc/configuration.txt b/doc/configuration.txt
index 9ca24361c..81a90daf9 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -4927,6 +4927,10 @@ http-after-response <action> <options...> [ { if | unless } <condition> ]
   There is no limit to the number of http-after-response statements per
   instance.
 
+  Note: Errors emitted in early stage of the request parsing are handled by the
+        multiplexer at a lower level, before any http analysis. Thus no
+        http-after-response ruleset is evaluated on these errors.
+
   Example:
     http-after-response set-header Strict-Transport-Security "max-age=31536000"
     http-after-response set-header Cache-Control "no-store,no-cache,private"
@@ -5592,6 +5596,12 @@ http-error status <code> [content-type <type>]
   For this reason, the errors continue to be returned even when the process is
   chrooted, and no file change is considered while the process is running.
 
+  Note: 400/408/500 errors emitted in early stage of the request parsing are
+        handled by the multiplexer at a lower level. No custom formatting is
+        supported at this level. Thus only static error messages, defined with
+        "errorfile" directive, are supported. However, this limitation only
+        exists during the request headers parsing or between two transactions.
+
   See also : "errorfile", "errorfiles", "errorloc", "errorloc302",
              "errorloc303" and section 3.8 about http-errors.