Jim's Helmet

Developer outfits only exist inside Treasure Bags, which means you need a world set to Expert or Master, since bosses do not leave bags at all in Classic. From there any Hardmode boss bag can produce it, with the one Queen Slime drops as the single exception. The roll is uncommon, so it tends to turn up by accident partway through Hardmode rather than at the end of a deliberate hunt.

The clothing arrives as a unit, so there is no chasing a missing piece. Put the lot in the social slots and your character picks up a glow plus a trail of after images while moving, the same two effects Spooky armor and Ninja armor produce. Both are appearance only and neither touches your stats.

Jim's Wings come along with the clothes and are the one part that actually does something. They behave like the Hoverboard in that they stay hidden until you use them, so they will not sit on your back spoiling a costume while you walk around.

For a long stretch the set existed in the game with nothing dropping it, and back then wearing any part of it slowed you to a crawl and stacked debuffs on you. That was a deliberate lock, and it came off when the set was made a real drop.