We might just have a new contender for the title of most Borderlands boss in the history of Borderlands bosses.
Spoiler alert for those who've rejected the infinite communication potential of the Internet in favour of scouring games for days, months, even years, in search of every last secret. (More power to you! I'm just impatient.)
Here's the skinny: near the beginning of Borderlands: The Pre-Sequel, there's a (very funny) side quest in which you fulfil a dead man's last wishes by walking up to a Scav named Nel and telling him he's a d**k. He slumps to the ground, casts his gaze at an unforgiving sky and bellows, "NOOOOOOOOOO", and everybody has a delightful time. That, as far as anybody knew, was that.
But as this video from Prestige'r MadLuigi demonstrates, Nel is also a boss-level enemy. He just takes a little... goading to bring out of his invincibility shell of passive-aggressive depression.
The short version? You need to do some wickedly specific platforming to assemble a series of letters -- D and K -- to hang on a building in Nel's direct view. With a little creative placement, the building, once stamped "IC 57", reads "D**K". At that point Nel loses it and the battle, which also includes a bunch of other enemies, begins.
For your troubles you get a legendary sniper called the Chikamin Skullmasher. You might recognise said pulveriser of brainmeats from Borderlands 2, which included a very similar gun.
So there you go. Vandalise a building with a crude insult, blast a very angry (and probably a little embarrassed) man to smithereens, and get a neat gun. Borderlands in a nutshell.