diff --git a/SC2139.md b/SC2139.md
index 3335b49..7d962b0 100644
--- a/SC2139.md
+++ b/SC2139.md
@@ -18,6 +18,14 @@ With double quotes, this particular alias will be defined as `echo /home/me`, so
 
 By using single quotes or escaping any expansions, we define the alias as `echo $PWD`, which will be expanded when we use the alias. This is the far more common use case.
 
+Note that even if you expect that the variable will never change, it may still be better to quote it. This prevents a second round of evaluation later:
+
+    default="Can't handle failure, aborting"
+    trap "echo $default; exit 1" err
+    false 
+
+The trap now has a syntax error, because instead of running `echo $default`, it runs `echo Can't handle ..` which has an unmatched single quote. Avoid early expansion unless you're equally comfortable putting `eval` in there.
+
 ### Exceptions
 
-If you don't mind that your alias definition is expanded at define time, you can ignore this warning.
+If you don't mind that your alias definition is expanded at define time (and its result expanded again at evaluation time), you can ignore this warning.