Played Stellaris again, just to bug hunt. Found the Abandoned Terraforming, which is not very funny just fucks with people's OCD, where you just cant finish it and it just stays in the situation log and on the map, its apparently common this is patch. Found the much more entertaining one, where if you decide a genocidal war (anyone can, because the tech Existential Expulsion gives you the total war CB "Wipe Them Out) against a set of enemies which includes a nomadic one, disable their worldship but white peace out before everyone is defeated, you can end up with an arkship and it is functional, just lack the techs to be really useful. You also cant delete it.
The real hilarious one works as a great hard lose too. First, you need a Hegemony-type federation that just got its ass kicked and hegemon annexed in a total war. Second, have an awakened Benevolent Interventionist empire angry at you(insults, terror bombing, etc.) and declare on you. Just as you accepted the surrender of the federation, accept subjugation to the FE. Then, because they are nice guys, they decide to join the federation as well and immediately become its president, dragging you along with them. Now you cannot leave, because you cannot declare war on your overlord to break free of servitude because of the federation, but you cannot leave the hegemony type of federation without the approval of your president, who is your overlord. Would be a softlock already, but add the redditoid strongwomyn crisis to the mix, who will start by declaring war and destroying all FEs and she cannot be attacked until the event line progresses enough, and you will be dragged into a war, so fleets will be summoned you out of thin air until you lose, as the crisis cannot be ended.
There is another softlock in that questline, where if you fight and kill her colossus too early, then it becomes impossible to finish the event and the crisis progresses indefinitely.