diff --git a/code/__DEFINES/subsystems.dm b/code/__DEFINES/subsystems.dm
index 9ee7f5db7e..81b5c5620f 100644
--- a/code/__DEFINES/subsystems.dm
+++ b/code/__DEFINES/subsystems.dm
@@ -146,6 +146,7 @@
 #define SSAIR_EQUALIZE 8
 #define SSAIR_ACTIVETURFS 9
 
+// |= on overlays is not actually guaranteed to not add same appearances but we're optimistically using it anyway.
 #define COMPILE_OVERLAYS(A)\
 	if (TRUE) {\
 		var/list/ad = A.add_overlays;\