I have noticed a recurring glitch lately. Here is what usually happens: I am running around as gunner killing pros. The pros on my team let the assassin juice. The assassin obviously comes after me at the end of the juice run. Usually I slam them away (if I have it available) and attempt to finish them. I had this happen both as a gunner and as an assassin. I emptied half of a minigun clip on an assassin, and she just wouldn't die. I ended up getting slashed to death after the daze wore off and an assault weakened me enough to let the assassin finish me with a front grapple. After I died and respawned, I could damage the assassin normally. I also had this happen to me as assassin the other day. I was juicing and destroying a few turrets and this gunner tries to slam me. I jumped and I am almost positive I missed the slam, but I was dazed. I wasn't flung at all though. I lunged right at his miniguns and let him shoot me for a good 3 seconds. I took no damage whatsoever. I then slashed and grappled him, but during the grapple animation his team mates started shooting me and I died. However, I watched my dead body complete the grapple animation. I had my katana and I front-grappled the gunner. I died and it did that zoom out thing that shows my dead body. The body disappeared, but I could see the katana slash motions. In the kill feed it said I killed the gunner after I died. I have had this happen several times lately. I think it is a mixture of lag, framerate issues, juice, and the use of the slam ability.
If true, this may possibly be the way to reliably replicate the "invincibility glitch" that happens a lot randomly by accident to assassins and tanks when juiced. Maybe the devs can use this, they were quite willing to reward anyone for this information a while ago.
It's just lag. No glitches or any other nonsense involved here. As far as seeing the Assassin grapple slashes even after you die and getting the kill, the first slash probably killed him. Sometimes if you die at the beginning of the grapple animation, the rest of the slashes still show, but don't do damage.