The London Metal Exchange market for nickel futures broke last Tuesday: Nickel prices shot up due to margin calls and forced buying by some big short traders, and in response the LME canceled $3.9 billion of trades, rolled back the price to pretend the Tuesday morning session had never happened, and shut down trading for a week. (We talked about this here and here.)
This morning the LME reopened for nickel trading, but quite gingerly. LME announced that it would only allow the price to move up or down by 5% from its previous close. That close was itself sort of artificial (since it ignored all the canceled trades from Tuesday).