I never really noticed this, so I'm gettin input from everyone else before I start making assumptions, but a spy was neutralizing an ND133 in 116 on Hospital, and I was behind my partner Merc (who had 1/2 health) tellin him that "Go flush him out, I'll watch the exits).
I see the Merc go in and the spy jumps on him, killin him. I went in and tazed the spy.
Right after tazing him (when the spy was standing, animating that he was gonna fall to the ground), I put 30 rounds (literally, the dead Merc was watching this all and confirming it) into him.
No death. The Spy then falls to the ground, and I unload another 15 or so (did this quickly because I had to get back to the other floor to cover the other spy), and that did the trick.
Quickly took the ammo bin in 116 for a reload (sure enough, I had 15 in clip and 90 in reserve.) and I moved on.
So are spies invincible right after the taze? Do you gotta wait til they hit the ground?