I'm on a role here!
I'm pretty sure I made a post about this some time ago. Since the release of 3.0 final, there's been a bug with lastinv.
It's pretty simple - if a weapon/ability slot is selected more than once, lastinv no longer functions. I believe it tries to switch between 2 instances of the same weapon slot. This is rather annoying as aliens in particular since you don't have crosshairs and in intense battles, you cant tell which ability you have selected (ie blink/swipe and leap/bite). I use lastinv combined with fastswitch a hell of a lot when attacking and often end up dead because of this problem. I don't want to turn to scripts because of the principal, so I'd like to see this issue addressed in the next release of NS (being 3.1).
It only cropped up after the release of 3.0. So I know it can be fixed. It just needs to be bought to your attention.