Okay, basically there's an issue where crafting level 400 stuff still grants XP...
The original thread was removed, but did have a dev response prior to that.
But the Internet never forgets.
Josh Davis, a dev, commented on the post prior to its removal, sourced from reddit
This is not intended behavior. We should have a fix coming out soon to correct the issue. In the meantime, I would avoid continued exploitation of this bug.
Obviously, there are some issues with this, as it is apparently easy to use to grind XP with cheap 400 recipes for levels, which translate into skill points, which are character-bound currency for purchasing things for the Legendary and usage of the Mystic Forge.
Response from Chris Cleary in the reddit thread
The post on the forum was removed because advertising or talking about exploitation is strictly forbidden on the official forums. While the issue at hand is a game bug, and is being addressed, it should be noted that abusing a game bug for gain is considered an exploit. I also understand that most users are going to run into this during normal gameplay, and I am fully taking this into account when investigating players abusing this bug for significant gain.
Edited by MazingerZ, 06 September 2013 - 09:13 PM.