It's not really one glitch, it's a result of many, many, many glitches all tied to one buggy game mechanic. Basically, if you get Relm to sketch something that was not meant to be sketched (IE Gau when he shows up on the Veldt, and I think I remember hearing about Intangir) strange things will happen. You might get more items than can possibly be obtained through normal play (99 Sky Renders comes to mind, or 6x99 daggers), or it might screw up your game save. I heard some but not all of these were fixed with the re-release on Anthology. You can find a list of sketch bugs on GameFAQs I think.