diff --git a/hardware/sejfduino/ERRATA.md b/hardware/sejfduino/ERRATA.md
index af8a338..6b3e04c 100644
--- a/hardware/sejfduino/ERRATA.md
+++ b/hardware/sejfduino/ERRATA.md
@@ -9,5 +9,11 @@ in a pull-up configuration between reset line and ground.
 Sensible place for that is between one C2 leg and reset line going next to it
 to J6 programming header.
 
+### #3: RS485 transceiver input/output enable is floating by default
+This breaks programming serial communication, unless input/output enable pins
+are properly set.
+
+This is fixed by applying optiboot patch - `0001-Disable-RS485-transceiver-when-running-the-bootloade.patch`
+
 ## v2.1 - "pulled-up" green PCB
 Errata #2 fixed.