diff --git a/MISTAKES.md b/MISTAKES.md
index 85d3be9..bc52033 100644
--- a/MISTAKES.md
+++ b/MISTAKES.md
@@ -5,3 +5,9 @@
 You WILL frick up and your binary encodings WILL overlap.
 You definitely need some tool to design instruction opcodes
 for anything more complicated than just 1-byte instructions.
+
+## Load/store instructions MUST be able to use register as address
+
+I have no idea how we didn't notice this mistake.
+I guess it is not too bad for a CPU with 16 bytes of memory,
+but for something bigger it is unacceptable.