Watching movies while playing is not a bright idea.
Changing armory and not checking your gear and cube is not smart either.
Talking to your wife or girlfriend is another one while playing, especially if she is trying to upset you or is upset because you are playing all day and she wants attention.
I would only add to that: ALWAYS GO TO TOWN before pausing the game.
Your post made me remember a couple others:
Upon towning at end of bounty:
Do Not go AFK until completely in town.
Iāve ripped more times than I care to say from assuming my towning is assured halfway through the portaling.
After finishing a GR, get your drops and upgrade your gems and town quickly.
Too many times there are undead monster like bats and crawling monsters etc. that will one, two or three shot you.
I agree with this. One thing I would recommend is a player starting HC should make a character in SC and treat is as HC character so they can learn survival a little bit before going out and making glass cannons to blast through high level rifts.
Playing HC while in a zoom meeting. The chair of the zoom meeting did something and my screen minimized in the the middle of a fight. Double procāed. Didnāt die but lesson learned.
Game crashes and server lagg. Those two have killed me most of all.
fmod is the biggest killer lately. Itās terrible.
Some some types that surprised me and Iād be extra cautious with:
Those the ones for me, also there are certain traps that are nastyā¦ Surly there are more threats, but like most already mentioned, all off the above are 10 times more dangerous when going into a rift you think you can clear easily, but donāt have the defensive stats to back it up. I feel that understanding how your defense works is key.
Alcohol may be an issue with me since I do play drunk maybe once per week and it does hurt my survivability, but Iām happy when I die
To be really honest, this guyā¦
ā¦scares the hell out of me.
Story time:
Years ago I was on a wizard (used to be known for playing that class), seasonal hc of course, I decided to join a split bounty pug maybe t8 or something (before t16 was introduced) and chose act 5 because it was the only act open (anyone with good sense avoids that place). I got to malthael, I paused and wondered, āam I really geared for this guy?ā Thought about asking for help but then decided (sober), āif I need help, I have no business doing split bounties at this levelā. My wiz had enough firebird items to get the second life from that and I had the unstable anomaly passive. I was like YOLO and dove in without saying anything to the other 3 players.
I was just me and malthael in a fight to the death. I started off doing OK, but then he clipped me with something and I. PROCāed. I procāed my cheat death on just some minor attack and all I could think was TIFU My pulse went through the roof.
Then I procāed my second cheat death.
My heart rate, which was already through the roof, just found a new fricken level and then kept climbing. The fight went on and on and on and on. I am not exaggerating that it was a minimum of 10 minutes. I cheated death 5 or 6 times.
I was scared like nothing Iāve ever felt in my entire life. Not because I was going to die in a video game but because I thought that I was about to have a god damn heart attack for real.
My wizard ended up dying, but I didnāt. The next day I made an appointment with a cardiologist and promised myself if I ever felt that way again I was just going to walk away from the computer and let my guy die,
I consider Malthael to be 1-2 levels higher than the rest of the enemies.
I cringe when the Season Journey requires killing him on TX.
I donāt usually have a level 25 Gem of Ease yet.
I have lost a few HC Season heroes wearing 4 gift set pieces.
I can say without doubt, that I have successfully killed off every hardcore character I have had and not on purpose.
The greatest threat is yourself in my opinion. Never be lulled into a false sense of security no matter how strong and damage resistant your character is.
Also from my perspective only, I found that to truly play HC well, you cannot be playing non HC characters at the same time. That contributed to 50% of my deaths by switching characters. Focus on HC exclusively and do not rely on a cannot die follower with cheat death and the passive cheat death to save you at higher levels because if damage is high enough, it can not only trigger both, it can be more than both combined and kill you instantly.
(And learn to run very fast to an exit)
EDIT - If you are not doing a Greater Rift, always use goldwrap and Boon of the hoarder gem maxed. 
- This way unless you lose concentration, you can only die in a GR. Also do not forget to change out of your gold gear before the GR. That is a killer too.
Donāt attack Sand Dweller mobs either, they reflect dmg similar to Dune Dervish.
Overconfidence and/or familiarity.
I find it very similar to people who rock climb, normally when you hear of people who die rock climbing its veterans who make a stupid mistake. New climbers are tripple checking everything and still very nervous.
My first season of HC way back when I managed to go the entire season without losing a character. Now when I lose one its because I became less afraid of dying. I make silly mistakes in pushing too soon, or not paying attention enough.
Everyone else already covered the mobs to watch for. And its HC so you will die eventually, there are scenarios that you just cant avoid and survive. But if you go into each rift a little scared it will benefit you more than anything else IMO.
Back from like a 5 year break! Playing solo so far; cleared GR123 and no deaths so far. My (re)observations of things to watch out for in no particular order: (Iām Rathma necro).
There are of course many other things that can kill you fast but the above is my list of things which are a lot more dangerous than you might think. Iām probably forgetting some things.
If youāre playing solo always run 2 good cheat deaths (templar is best, enchatress is worst, scoundrel might be okay).
You can play somewhat recklessly until one cheat death procs - obviously try to avoid it but you need to take some risks to push higher).
After one procs start playing much more carefully or go to town if you can afford it.
IF and after your second cheat death procs, IMMEDIATELY abandon the fight and go to town from a reasonably safe location. I donāt care how close I am to whatever goal - just assume that a second proc means youāre about to die permanently if you do not retreat. Countless things can kill you in the blink of an eye at this point if you screw up or get unlucky - nothing is worth dying.
Always have an escape plan in mind. Know which way you will run and Ideally have a path clear of mobs which is not always possible when pushing high GR and itās not worth the time killing all the stragglers. Nonetheless have a plan in mind and homing pads on the follower to help you portal out.
If youāre playing solo, at the first sign of lag press escape ONCE and continue clicking away from the mobs. This will pause the game as soon as possible while you keep running away - giving you the best odds of survival when the game catches up. Turn OFF autofriend join.
When entering any dungeon/rift/waypoint with a load screen immediately press escape ONCE during the load screen. This will ensure that the game starts paused once itās done loading and before you actually see whatās going on. Will save you if there are enemies right near the door which happens sometimes.
Final advice and personal opinion / decision to make. Prioritize MORE toughness in hardcore. All he guides on icyveins, maxroll etc are meant for softcore and are basically glass cannon. Thatās a recipe for disaster. You wonāt push as high as them but you will live a lot longer. Most things in the game should NOT be one shorting you except stuff like explosions or big telegraphed attacks that you must dodge. If you are proccing left and right then your DPS is way too high relative to your toughness. Ideal balance is being able to kill stuff reasonably fast AND being able to reasonably survive among same stuff. Again this is my personal conservative opinion - some people like to go allin on dps and hope nothing hits them.
And getting PK in town by griefers. Stay out of act 1 or get killed by randos.