Every time you take an item out of the optibench, it gives you xp. If you give the optibench the item it is trying to craft, it will immediately move it to the output slot where you can take it out for xp. Repeat for infinite xp.

The item I was crafting was tinkers construct glass into minecraft glass. I don't seem to be able to reproduce the issue with other recipes.

This is indeed a bad one. It will work with any recipe the vanilla furnace also has (so one iron block to 9 iron ingots will also work as you can melt iron ore to iron ingots).
I am not sure how I can fix this without breaking the optibench. Then again, should the player gain XP from crafting as there are many ways to craft somthing and back.