diff --git a/base/expr.jl b/base/expr.jl
index c37f1a6482162..e45684f95a34f 100644
--- a/base/expr.jl
+++ b/base/expr.jl
@@ -513,6 +513,13 @@ The `:consistent` setting asserts that for egal (`===`) inputs:
     even for the same world age (e.g. because one ran in the interpreter, while
     the other was optimized).
 
+!!! note
+    The `:consistent`-cy assertion currrently includes the assertion that the function
+    will not execute any undefined behavior (for any input). Note that undefined behavior
+    may technically cause the function to violate other effect assertions (such as
+    `:nothrow` or `:effect_free`) as well, but we do not model this, and all effects
+    except `:consistent` assume the absence of undefined behavior.
+
 !!! note
     If `:consistent` functions terminate by throwing an exception, that exception
     itself is not required to meet the egality requirement specified above.