Warn about the risks of using catch-all patterns (#200)

book/src/05_ticket_v2/02_match.md

@@ -68,3 +68,10 @@ match status {
 ```
 
 The `_` pattern matches anything that wasn't matched by the previous patterns.
+
+<div class="warning">
+By using this catch-all pattern, you _won't_ get the benefits of compiler-driven refactoring.\
+If you add a new enum variant, the compiler _won't_ tell you that you're not handling it.
+
+If you're keen on correctness, avoid using catch-alls. Leverage the compiler to re-examine all matching sites and determine how new enum variants should be handled.
+</div>