diff --git a/doc/limitations.md b/doc/limitations.md
index 6470f65a2..07789fa88 100644
--- a/doc/limitations.md
+++ b/doc/limitations.md
@@ -335,3 +335,38 @@ the splat operator (`*obj`).
 This is a deliberate trade-off: implicit conversion forces every
 coercion site to go through method dispatch and can silently mask
 type-mismatch bugs.
+
+## Nested `def` in Singleton-Method Context
+
+`def` written inside a singleton method (`def self.foo`) is placed
+on a different class in mruby than in CRuby. CRuby registers the
+inner method as an instance method of the lexical enclosing class.
+mruby registers it as a method of the enclosing receiver's
+singleton class, which makes it visible as a class method of the
+enclosing class.
+
+```ruby
+class SomeClass
+  def self.class_method
+    def nested; 'nested!'; end
+  end
+end
+SomeClass.class_method
+```
+
+#### CRuby
+
+```
+SomeClass.nested        # NoMethodError
+SomeClass.new.nested    # => "nested!"   (instance method)
+```
+
+#### mruby
+
+```
+SomeClass.nested        # => "nested!"   (class method)
+SomeClass.new.nested    # NoMethodError
+```
+
+Writing nested `def` like this is unusual; this difference rarely
+surfaces in practical code.