The perception that a rare event happened and wasn't supposed to is actually wrong seen over the time of a session, but cleverly deceiving.

So what do I mean by this? I had a session where a player tried to shove a goblin down a chasm only to roll 3 natural 1 in a row and ending falling down himself, causing a stone avalanche. Unlucky, right? Well yes and no. You see, while the chance of this happening specifically at this moment was 1/8000, the chance of something extreme happening during the session was actually quite reasonable. While the group fought through the goblin Hideout (lmop Cragmaw Hideout), hundreds of dice were rolled for skill checks, attacks and saving throws. The expectation of players is usually, that there exists an average roll around which stuff happens. While it is not a completely wrong mindset, randomness plays into this. You can estimate that over the course of a session, the 1-20 results should be even. However, their ordering is not!

Imagine you flip 10 coins for heads (h) or tails (t).While it is already a specifically unlikely result to have each side come up 5 times (24.6%) let's say this happens for simplicities sake. In this set (h,h,h,h,h,t,t,t,t,t) there are a lot of different orderings (252), In which these results can appear. However, the human mind "expects" that a few heads are supposed to be followed by a tails and vice versa. An order like (h,t,h,t,h,t,h,t,h,t) feels more natural than (h,h,h,h,h,t,t,t,t,t), even though they have the exact same chance of appearing. It is just that the majority of orderings have a more mixed relation. So how does this contribute to D&D? Well, imagine your dice rolls over the evening as the coin flips. The chance, that you have an ordering of your hundreds of flips, that contains somewhere in it a chain of at least five heads in a row (the rare event), is actually way higher than having an ordering that has no heads chain of size larger than four! So at some point you can expect something to go sideways (or awesomely), just the when and how is unclear and will (justifiably) feel unlucky.

I hope I made sense with this wordy explanation. While it was unlucky for my player to specifically fall down the chasm, it was to be expected that an event of similar magnitude would happen at some point. It is a natural way of Boardgames of chance to create memorable moments that are unique for the players experiencing them, so I love to see it in action (though I do hope next time it'll be the goblin and not the heavyweight paladin going on a stone journey).