diff --git a/wp-content/themes/twentytwelve/style.css b/wp-content/themes/twentytwelve/style.css
index fb5d3c2443..5239637793 100644
--- a/wp-content/themes/twentytwelve/style.css
+++ b/wp-content/themes/twentytwelve/style.css
@@ -317,6 +317,7 @@ img.wp-post-image {
 	box-shadow: 0 1px 4px rgba(0, 0, 0, 0.2);
 }
 .wp-caption {
+	max-width: 100%; /* Keep wide captions from overflowing their container. */
 	padding: 4px;
 }
 .wp-caption .wp-caption-text,