Tested with Google Chrome Versión 23.0.1271.64.

If you have a resource activity on the course front page indented to the right at least three times. Whenever you click on the left identation arrow the ajax effect decrements two numbers in the css class.

For example If you have idented three times to the right then the div that contains the activity has the class mod-indent-3. If you click on the left identation arrow it must decrement only 1 to that css class and put mod-ident-2 but it put mod-ident-1.

On firefox this works as expected.