SharePoint Developer Question:

SharePoint 2013 provides Support for both 14 Hive and 15 Hive. Both the directories are Created by default. So if you deploy a SharePoint 2010 Solution that refers to say _layouts folder, the reference will be made to 14 Hive and not 15 Hive by default. To make the solution refer to 15 Hive’s _layouts folder you need to Re-Create the Solution in Visual Studio 2012 and fix the _layouts reference to add ’15’ into it.

e.g.
14 Hive – _/layouts/custompage.aspx
15 Hive – _/layouts/15/custompage.aspx

Notice the reference of 15 in _layouts folder reference.