I've made a small list of ways I've found, can anyone add on? These have actually happened to me before in-game BTW.
1) Fall in a pit of bedrock walls near the Nether or Overworld bottom, while carrying nothing you can use to jump-stack, climb, or otherwise escape. No way to die due to not being in hardcore and not having any self-lethal items.
2)Teleport to the bedrock ceiling intending to build a portal there, but forget a diamond pickaxe/extra obsidian and mess up the portal. No way down, no way out. No way to die due to lack of items and no mobs and not being in hardcore.
EDIT: using 'legitimate' cheats/glitches - pearls, minecarts, etc.
3) Ender-pearl into a space in the bedrock layers and discover you can't fit out while having no extra pearls, water, etc. but you're not suffocating. Worse in my case as I had gravel to suffocate myself with so at least I can restart, but I messed it up by jumping up on the second gravel, leaving my head exposed but body buried, and then one of the gravels became flint when I picked it up (did not have silk touch shovel).
Any other ways 'true death' has happened to you in survival mode?
EDIT: RFlowers adds:
4) Similar to 3), elytra-fly through a one-block hole in bedrock into a space to small to fly back out of.
I also have another one.
5) water into a bedrock area including such a 1x1 hole, then sponge it after swimming through the 1x1. No way back, and the wet sponge can't help you. U
6) Following 5: Using an empty bucket instead and then chucking it through the hole also counts. Of course, 5) is more likely to actually happen in gameplay unless you carelessly press Q in the exact wrong direction after swimming in.
These things can happen (and have happened to me) when either building a wither in the nether ceiling, mining diamonds near bedrock, or trying to build a wart farm or mushroom farm on the nether ceiling. So they may seem unrealistic, but they are possible if you have certain intentions and want to take advantage of bedrock. Notice that all the methods of getting stuck involve bedrock so far.
Even in hardcore mode you can access cheats by opening the world to LAN and teleport, fly or break the bedrock.
The closest I've gotten was when I was stuck in a 1X2X1 hole with a villager I was transporting to my base, the hole was just in dirt but I was unable to dig, all I could do was open the villager's trading window or hit him.
I could have killed the villager to get out without cheating and gone back to get another villager but I chose to activate creative mode and fly out of the hole, I always play with cheats turned on though this is the only time I can remember using them in a survival world.
(Apart from one time when I wanted some screenshots of the area around my home base for a forum thread and was way far away at my ocean base, so I activated creative mode, teleported home, got the screenshots, teleported back and switched back to survival mode.)
As long as you have Hard Mode - no Hardcore required - you can die if you can Jump (which uses the Hunger Bar - slowly - but more-or-less eventually you starve). I've probably used this about half the time.
But the worst is Hexalobular's, above. I basically mark and /or label All holes I can fall-into, partially because this 1X1 X2 hole problem (if it's Really bad, realize as-in the-above, the ceiling is just-above so you Can't Jump [though technically this would have to be Obsidian and /or Bedrock, everywhere], so you can't even use the method I just described) is impossibly-annoying.
So thank gosh it's not just " /gamemode creative ," but lately " /gamemode spectator ," to get (fly) us out or phase-through Blocks. Otherwise, " /tp [X Z -Y] " is the only way.
I tend even to want to free Mobs so that I don't accidentally Get Stuck In There With Them. Ahead of time, I mean (since one Can't, when one's-Stuck).
I got stuck inside my mob grinder when I accidentally fell in. I was able to keep the horde at bay, but because we were all being pushed by water I had zero time to actually break any blocks...not that it would've mattered, the only options were a long fatal fall into the killing chamber or a long fatal fall from dropping out of the hole I chopped in the wall.