This bug has been in the game for quite some time I can imagine so here's an example and some details:
The "Pentagram" ability during the Samael fight bugs out *sometimes* when paired with his "Fire Circle".
From what I understand, it is supposed to give you a nice flat amount of physical damage reduction (511 is my best guess) while standing inside the icon until the ability is over and it does a very good job at keeping you alive throughout the fight. You lose the buff if you move or teleport out of the safe zone as intended.
However, if Samael casts his "Fire Circle" while you're standing inside the "Pentagram", you lose the buff it's supposed to give you and you end up dying like 9 out of 10 times.
Here's a short video of this happening with a step-by-step explanation of what's shown on the screen:
First of all, this is single player softcore, so no lag or desync is involved, although I can imagine it could even be worse on multiplayer.
There is a function on d2stats that lets you read stats without tabbing out of the game so I bound it to "V" and I was spamming it while I was recording this.
So what happens is, Samael casts "Pentagram" so I enter the safe zone and d2stats updates my physical damage reduction to 511 in that case, but while the ability is still ongoing, Samael decides to cast his "Fire Circle" on top which leads to my character losing the bonus despite still standing on the icon and as expected - death follows shortly after.
Now, I'm not 100% sure if this is an actual bug or if this is intended, but it is extremely punishing and if (when) it happens, you have barely any time to react so your best bet is to build something to counter it like avoid stacking or some physical damage reduction yourself (my assassin with 60% chance to avoid managed to live through this last season but if it was any other hardcore character on the server, it would have been a certain rip)