diff --git a/include/openssl/ssl.h b/include/openssl/ssl.h
index 8b59ea5..b123b34 100644
--- a/include/openssl/ssl.h
+++ b/include/openssl/ssl.h
@@ -3460,7 +3460,8 @@
  * |SSL_get_client_CA_list| for information on the server's certificate request.
  *
  * Use |SSL_CTX_set_cert_cb| instead. Configuring intermediate certificates with
- * this function is confusing. */
+ * this function is confusing. This callback may not be registered concurrently
+ * with |SSL_CTX_set_cert_cb| or |SSL_set_cert_cb|. */
 OPENSSL_EXPORT void SSL_CTX_set_client_cert_cb(
     SSL_CTX *ctx,
     int (*client_cert_cb)(SSL *ssl, X509 **out_x509, EVP_PKEY **out_pkey));