The standard Windows Alt-TAB behavior stops working altogether when I include defaults.ahk. If I edit defaults.ahk to comment out Tab from the list of combo keys, it works normally again.

I get this problem without configuring any specific keys, using this as my AutoHotkey.ahk:

SendMode Input
#NoEnv
#SingleInstance force

#Include <dual/dual>
dual := new Dual()
#Include <dual/defaults>

*ScrollLock::dual.reset()

I'm on Windows 8.1, AutoHotKey 1.1.19.02, dual from github trunk.