Warning! Improper handling of this information may result in the permanent lost of your items.
Initially started as partial bug of Katovic questline, it became a hidden game functionality useful for those who want to keep their inventory optimized. I am talking about Ryrin's Heart located at the Frozen Plain E6. You can drag any item including premium items except for quest item into Ryrin's Heart and it will be lost permanently. Even though it asks the quantity of items to be dropped, it only takes one at a time so it may take a while to get rid of bulk items.
The Symphonia patch also addresses the Ryrin's Heart by adding it to Trade Password Protection functionality and as a side note, the shelf besides the "Rescue Knight Material Administrator" also functions like the Ryrin's Heart but was fixed in a later patch. This adds to the legitimacy of Ryrin's Heart as a de facto trash bin for undiscardable items since it wasn't fixed along with it.