In Firefox's firebug, I noticed two versions of Jquery is loaded. The first one is the one I specified in the theme.liquid, version 1.11.1 and the second one is injected by shopify, version 1.10.2.
It appears the 1.10.2 version is injected from {{ content_for_header }}
How do I remove the injected version? I noticed several shopify's shop have the same problem. Some sites have 3 or more versions of jquery included.
including multiple version of jquery can potentially cause problem