These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: The Inflatable Sea Monsters in Duke Caribbean relentlessly barrage you with missiles the millisecond you come into sight, usually from around a corner and with absolutely no warning. Thankfully, they go down with only one shotgun blast, but expect to do lots of Save Scumming.
Ear Worm: "GrabBag", the title music of DN3D. An MP3 of it is available for download from the composer's site.
The Protozoid Slimers, which hatch from eggs, hide in vents, and climb up on your screen, tearing you apart unless you kick 'em quickly. They're pretty hard to hit in the open, too. Luckily one hit of anything kills them if you can connect, including the otherwise damage-less Shrinker.
Sentry Drones. They explode on contact, they often hide right behind doors or around corners, they can take a lot of damage, they make irritating noises, and they're everywhere.
Literally appear in Lame Duke (at least episode 2 level 1), as this beta version features a bat enemy which was dropped from the final game.
In v1.3D, you could select the Mighty Foot as a weapon and then kick with the melee button, at the same time! Result: Duke somehow manages to run around while both of his feet are on screen, whacking enemies. (Un)fortunately fixed in later versions.
The Build engine is notorious for its flaws that allows mapmakers to come up with interesting map ideas. Bizarre takes this fact and drives it straight into Mind Screw territory.
There is the possibility that you can die as you're exiting the level. If that happens, you start the next level with your stuff intact and 100 health.
In the Playstation Vita version, the final level of The Birth has an area where you can clip out of bounds with the help of those rotating gear-shaped things in the water. This counts as completing the level even though only the first half was completed, and will even give you the trophy for beating it. However, you'll be sent back to the cutscene from before entering the first level.
Moment Of Awesome: At the end of episode 2, before fighting the boss, Duke states that he'll rip off his head and shit down his neck. Cut to the post-boss battle cutscene where Duke follows through on the promise.
Only The Creator Does It Right: Nuclear Winter is the least liked expansion pack, and the only one 3D Realms didn't make. It doesn't help that the first two levels are just remixed versions of ones from the base game.
Porting Disaster: The Tiger Game.com version. In fairness, it was actually one of the most advanced handheld games of The Nineties; there was just no way to replicate the Duke Nukem 3D experience on hardware barely equivalent to a Game Boy.
Less so the Mega Drive port. It does have a pretty decent framerate, considering how big the "first-person view" screen is, compared to other FPS games on that console, not to mention the save system which allows you to backup everywhere. Nothing else makes up for it properly, though: the whole port consists of 9 new levels, very loosely based off the Lunar Apocalypse episode, all done in a Wolf3D-like perspective, all peppered-up with barely audiable music and slightly messy controls. * And that's not even for Duke himself: in order to pause the game, you will need to not just press the Start button, but also X, Y and Z, all at the same time! Oh, and it didn't help that shortly after the game was released in Brazil (the only country that received a Mega Drive port), the whole thing got banned.
Episode 1, level 3 features a ridiculously-tricky segment where you have to jump and duck past a bunch of infrared laser sensors. If you hit one of the lasers, bombs go off and kill you instantly. Not to mention that some pig cops are on their way to kill you as well. And if THEY hit the lasers, it also triggers the explosions.