Hopper allows cheating XP

When I put the hopper block above a furnace and fill it up with coal (not charcoal - normal coal)
and the slots for fuel and unfried stuff are already full, the coal goes into the slot for finished/burned/fried things.

Then When I take the coal out of the furnace I get XP.

So it is possible to cheat XP in survival mode.
I would just have to re-put the coal into the hopper and get as much XP as I want.

It's the same with iron ingots, gold ingots, redstone dust, diamonds and emeralds.
But not with charcoal and nether quartz.