If a furnace minecart is pushing a hopper minecart through a powered rail (turned off) when it tries to turn, it glitches.
- Put down a few powered rails and don't turn them on
- Put down two normal rails to make one turn
- Place another few powered rails beside the other powered rails to make another turn
- Place a furnace minecart with a hopper minecart in front of it
- Start the furnace minecart