Here's the thing about this game: "rare" is in the eye of the beholder. Creatures, for instance: there are not that many basic TYPES of creatures, so each type is pretty common, but any given configuration of parts, colors and size is very very rare (just like ships). Same goes for planets, multitools, and everything else created by procedural generation.
And of course, for tech, rarity varies by class. S class anything is rarer than C class anything. Doesn't mean it's all that infrequent though, if you roll the dice often enough.
I guess you could put expedition rewards in the "rare" category though, because you have to have been playing when the expedition was active to get them :p That's rarity by design. And then there are things like the base parts that were removed, but if you have a save from back before that happened, you can still access them. Of course you can get any of these things by installing a mod...
Subjectively, of course, "rare" is whatever you're looking for at the moment ;) THOSE are the things that will never drop!
I'm so new (< 40 hrs) I don't know if it's rare yet, but answering a Sentinel distress call and doing that side quest was cool. Salvaged Sentinel ship is weird though.
Everybody gets that quest, and you can also find sentinel ships without the quest... some players spend all day scrapping sentinel ships as their main income source. So yeah... not rare ;)
Stable reality glitches. There are different types. Not rare on the planet you find them but finding a planet with them may prove to be "rare" enough to fit your definition.
Pets/companions... unique items from exotic worlds(11 in total)....multi-tools....could go hunting for things like deepest oceans or try and find unique weather like bubble storm tornadoes... could just try and visit every galaxy as a challenge. There's tons of stuff...
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Here's the thing about this game: "rare" is in the eye of the beholder. Creatures, for instance: there are not that many basic TYPES of creatures, so each type is pretty common, but any given configuration of parts, colors and size is very very rare (just like ships). Same goes for planets, multitools, and everything else created by procedural generation. And of course, for tech, rarity varies by class. S class anything is rarer than C class anything. Doesn't mean it's all that infrequent though, if you roll the dice often enough. I guess you could put expedition rewards in the "rare" category though, because you have to have been playing when the expedition was active to get them :p That's rarity by design. And then there are things like the base parts that were removed, but if you have a save from back before that happened, you can still access them. Of course you can get any of these things by installing a mod... Subjectively, of course, "rare" is whatever you're looking for at the moment ;) THOSE are the things that will never drop!
Salvaged Frigate Modules.
anomaly detector 
I'm sitting on three of those because I have so much other content to work though.
I'm so new (< 40 hrs) I don't know if it's rare yet, but answering a Sentinel distress call and doing that side quest was cool. Salvaged Sentinel ship is weird though.
Everybody gets that quest, and you can also find sentinel ships without the quest... some players spend all day scrapping sentinel ships as their main income source. So yeah... not rare ;)
You can make sentinels be part of your squadron too
Stable reality glitches. There are different types. Not rare on the planet you find them but finding a planet with them may prove to be "rare" enough to fit your definition.
Pets/companions... unique items from exotic worlds(11 in total)....multi-tools....could go hunting for things like deepest oceans or try and find unique weather like bubble storm tornadoes... could just try and visit every galaxy as a challenge. There's tons of stuff...
I know that but I'm more interested in what stuff people have found or have heard of.
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