Tested it a bit with the classic warp and incendiary ammo. Contrary to what Bioware said last year or something, it only glitch stacks when incendiary ammo is applied afterwards.
You could even do a warp then shoot incendiary then warp again, there will be 2 concurrent DoTs running - one glitched incendiary and one normal warp DoT.
*edit* In fact, if you shoot the target with incendiary ammo first before using warp, no incendiary glitch stacking with warp can occur until incendiary ammo runs out, no matter how many times you shoot or use warp.
This explains the somewhat "normal" performance of incendiary ammo against the possessed praetorian in this video, everything else was getting absolutely demolished.
Yes, indeed it does. Thanks for doing this, it clarified a point I always wondered about. Sadly, I'm afraid I'm guilty of having shared wrong information then... Since I always kept the line of "until proven otherwise, shooting or warping first, it's the same". Sorry about that, everyone 
Used to test it pretty extensively. It is 75% (at least on gold). It does not work when the phantom is in cover, flipping over cover, climbing ladders, somersault-dropping from heights, staggered/in stasis/etc. 1337hax shooters probably also know that it stops working for a brief moment at the end of a phantom side flip dodge. Works against everyone's(?) projectile powers and all host hitscan weapons.
Thank you for the answer... However, you're not telling me anything I didn't already know, sadly.
The problem is, even though the tests you guys did provide a good basis for what happens in my experience... The damage is not always consistent with them. Take a look at this vid by Djeck, for example. Very old, but still. He's soloing, and using a BW. He forces a Phantom into bubble and then gets a clean headshot... Result, one health bar of damage. Now. If we go by your tests... What should happen is that...
- he deals direct health damage, check
- he deals 25% of normal damage, no check here. Although there's no testing equipment available, the difference in the numbers is big enough that the bars of damage are a good enough indicator. Let's do some approximate math:
BW damage: let's say 1000. It's actually more with weapon passives and whatnot, but let's keep it simple. Headshot BW damage: 2500. Predicted Phantom health damage: 2500/4 = 625.
Now, platinum phantoms have 2194 health. So that means that each bar is worth 220, and that 625 should be worth almost 3 bars... Not just one. As I said, the difference is big enough for bars to be decent indicators.
I would not even pull this out if it wasn't for my experience agreeing with it, and saying that sometimes, seemengly randomly, the Phantoms' on host DR skyrockets from the "standard" (and still bullshit-ty) 75% to something even higher, which basically turns them into unkillable stabbing machines. That is, until you manage to catch her with her DR off. But still, it's *very* annoying when using a full auto weapon, which is not able to deal significant damage in that fraction of a second after she flips.
Lately, I've turned the DR off completely via coalesced.win... And still, sometimes, Phantoms seem to behave weirdly as far as damage taken goes, such as surviving a saber headshot with their barriers down.
... Am I seeing things? 