[DOC,FEATURE] Error message if permalink is not working anymore (e.g. http://…/?p=OldString although OldString does not exist (anymore))
[DOC,FEATURE] Developer Mode; (de)activate by pressing Ctrl + Shift at the same time

$.devMode() returns true if the developer mode is active

$(document).bind('devMode', function(e) { console.log(e.active); });
can be used to get notified of developer mode activation events.

[DOC,FEATURE] Permalinks are displayed in Developer Mode
[DOC,FEATURE] Small window notifies user if he is in developer mode

This feature is relevant for end users who accidentally activated developer mode.
This way they get to know how to exit this mode.