When selecting between slot numbers there seems to be a slight delay before you can select the next slot. I believe it is caused by forcing the weapon animations to play (so that you must wait until they are finished before selecting a new slot). This is most annoying when you go into battle expecting to switch to a certain weapon, when infact no switch has taken place. I.E. as fade, before going in to battle, I quickly press 2 then 1, so that I will have swipe in primary and blink in lastinv, only to find out later while trying to make an exit, that blink was not properly selected so lastinv remains empty.
To make sure this was not a bug in the HL engine itself, I started up HLDM and tested it out. It worked fine in HLDM, even after switching slots much faster than in NS the correct weapon was selected all the time (albeit sometimes with quirky equip animations) and lastinv was stored correctly.
To recap/reproduce (so far most noticeable while playing aliens):
1) Select a weapon by slot number, then select another slot in quick succession.
2) The weapon will not have changed from the first slot selected.
3) Lastinv will still contain the first slot weapon.
Its rather annoying waiting for/jamming a weapon slot key just to make sure you've got that weapon selected, especially while in combat (really, it should be as quick as select and fire). I'm not sure if this bug is new to 3.0 or has been around for a while, since up until the advent of +movement and all these BS servers, I would rarely access weapons via slot number.