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.
Level 33, On The Rocks. It's your first real Marathon Level, having you use a block generator to build bridges to get to tiny islands with the chips on them. It's exceedingly time-consuming because there's only a handful of generators throughout the level, and if you make one slip, you'll have to restart the level.
Level 73, Morton. You're not going to beat it if you can't trap the teeth within a spot from which it can't bother you anymore.
Level 89, Block Buster. The speed of the blocks that are replicated by the clone buttons makes them very difficult to dodge. And since Chip is a One-Hit-Point Wonder, he must never fail at dodging any of them.
Level 105, Short Circuit. A nasty maze of chip sockets and recessed walls, which forces you to memorize the layout of the maze.
Level 131, Totally Unfair, which requires you to memorize the layout of a previous level, titled Totally Fair.
Level 136, Doublemaze. It's two mazes shuffled together. There's a huge amount of chips to collect, you need every single one of them, and thanks to the level design (ice and thin walls) it's hard to tell which maze a given chip is in.
Level 147, Force Field. Pretty much any level that takes longer than 5 minutes, for that matter. Sometimes a level with a time limit of under a minute is even worse.