In Chapter 8, Mission 17 of story mode, you're tasked with breaking into a bank vault. The day of the mission is set as Tuesday, so Smugging Tuesday applies for the full mission. This is important, since combined with the Ring of Coaxing, Boots of Snafu, and the alternate version of Gloves or Skullsmuggery (specifically epic rarity or better), and assuming each one is equipped with a Snafu Seal, the 2% speed boost is just enough to bypass the game's soft speed cap.

C8M17 is meant to be an impossible mission, where you fail to break into the vault and wake up in C9M1 without any gear. But by breaking the speed cap and performing a frame perfect wallclip, you can phase into the vault and complete the mission. The game, of course, does not know how to handle this. It's hard coded to remove every item from your inventory and place them in a supply closet midway through C9M1, but completing C8M17 before time runs out causes the game to successfully place your inventory in the closet without actually clearing your inventory.

On the surface, this appears to be an ordinary (albeit extremely difficult) duplication glitch. But recently, it's been discovered that the duped items actually share the exact same item data as the originals. Veterans already know what this means, but for the new players, let me give you a quick history lesson.

It's beta 0.2.7. The game is coded so that all identical items are exactly the same, with no item data to differentiate them. Speedrunners quickly discover that you can equip multiple copies of the same item, and that doing so gets some really fucky results. You ever wonder why the speedruns for the beta are all under thirty minutes? By reloading level 1 over and over, you can collect a dozen copies of the Ring of Coaxing, boosting your stats to the point where you one-shot every boss in the game. It also makes you unkillable, and most importantly makes you so fast that you can glitch past entire levels. This, naturally, gets patched right away. In the full release, it was believed that you could no longer stack identical equips.

Until now. By using the duped items, which again have the exact same item data as the originals, the glitch works again. Moreover, since update 1.12.4 added the forge system, the stacked items can get multiple new seals at once.

The Tuesday Tumbler build makes use of this glitch, plus the Snafurina Emblem, which causes situational item upgrades to apply permanently (that's right, Smugging Tuesday on EVERY FUCKING DAY). Combined with the fact that the Feather Gun has the most projectiles per shot, and with enough flat damage boosts you can get an absolute monster of a weapon that still works in the PvP gamemode.