Secret/obscure in game events you know about

Recently, I learned in Cianwood, in HG/SS, if you only have one Pokemon, a random man in the Pokemon Center will give you a Tentacool. Infinitely, btw. This is a very small, unknown event, but what are some "secret" in game events you know about?

I also know about a lot of US/UMs missable side quests, and that Oleana looks for Rose in the expansion pass, but those are just a few.

I think it's a mix between obscure and well known but the slots of the Game Corners usually have different odds. The best slots for Gen I and II are well documented - and in fact, in Gen II, the game even gives you a hint to figure out which one it is without having to try them all.

Something a bit more obscure would be that there's a way to change the clock time in GSC.

If you're in Gold or SIlver:
- While on the title screen press DOWN + SELECT + B then enter the password, and play the game normally

If you're in Crystal:
- While on the title screen press DOWN + SELECT + B
- then release DOWN and B without releasing SELECT
- then press UP and LEFT
- then enter the password and play the game normally

The password is calculated according to your name, trainer ID and how much money you have - although there's a number of calculators out there for that, like this one

The Tentacool event is to help those who released their Pokémon from being unable to leave Cianwood since it requires surf/fly (latter of which can't be used unless you beat the gym), so you get a surf-ready Tentacool. 👍 I like how they think of these things!

Not sure if it counts but if the roaming Entei or Raikou use roar in frlg, they can't be encountered again after that. Learned that the hard way. :(

How about the Pokemon Ranch Phione? (or Mew for that matter, which was harder to get)

In X/Y there were these 2 random trades in Hotels that are also pretty forgettable/easy to miss.
https://www.serebii.net/xy/ingametrade.shtml ( at the bottom of the list)
mainly because the trades are bad and require talking to the person multiple times.
During playthroughs I always talk to everyone once and usually no more xD

I guess there might be quite a few who know about this now, but if you purchase Crystal on 3DS virtual console, you can play the celebi event after you finish the game and capture one legitimately.

IV played gen 4 so many times that its crazy but I've only just learned that Lucian can be found in the canilave library I can't remember what triggers it though

there is a dude hanging out w some alolan dugtrios in a town in usum :) this isn't rly that obscure like it's Right There where you can see it but this is just nice and pleasant and maybe i cried over it a little

okay but actual obscure stuff lol i don't know if this works in RS, but i know in emerald you can press i believe start or select in a double battle and it will show a number counter for damage instead of the typical bar. cause you know usually in double battles, it doesn't display numerical values, just the bar. but you can alter it. i might have the button wrong, it might be either of those with a combo of smth else. i never do it personally bc i'm Not a numbers kind of guy, but yeah.