A glitch (or possibly a leftover debugging feature) that allows you to upgrade any weapon to maximum in mere seconds. A complete walkthrough is below.
1. You must complete Paige's house and watch the event there BEFORE viewing the event at the Gaffer's buggy so that his inventory will not be affected.
2. Press triangle to enter the menu, then cancel out and enter the shop menu. Switch to your weapon attachments page and scroll down to the seventh row.
3. Move the cursor one space left to the Gaffer's inventory. Press the X button and Right on the D-pad at the EXACT same moment and you should get the "Broken Dagger". (The timing for this is quite tricky and may require a few tries). Place it in your inventory, then back out of the shop.
4. Attach the broken dagger to any weapon slot. Presto, you've maxed out the weapon's base stats, elemental attributes and anti-enemy attributes in one fell swoop. The game does show some useful weapon effects too (Poison, Stop, Durable and Abs Up), but these don't actually function as intended.
You can repeat this as many times as you wish to max out all six characters' weapons in a matter of minutes.
This also works if you just need some quick cash; you can tack the broken dagger onto some cheap throwaway weapon like a Kitchen Knife, then sell it for a nearly 4000% profit. (Thanks to soragamer72 for this bit!)
Pokemonfan4000's Youtube channel also shows off numerous other variations on this glitch, including ones that let you skip straight to the credits and get the Chronicle 2 sword at the start of the game. He also shows off some curious unused content in both Dark Cloud games. Really interesting stuff, so give it a watch!