The actual real world price for immediate delivery is called the spot price. The price of the future is just the price for a contract to have it delivered on a certain date.
As I understand it, Oil Futures come in two varieties, one kind result in you actually taking the oil when they happen, which is why that negative price craziness years back because if you're holding the futures and haven't got anywhere to put that oil that's a problem for you. The other kind though is some sort of cash equivalent, I guess maybe it resolves to the current spot price or something at the moment it stops being a future ?
So, for these mispriced Futures, what happens? If I buy $1M of Brent Crude futures and then just wait, and when my futures resolve that much Brent Crude would be worth $1.5M at spot prices do I... get $1.5M and somebody in the oil industry just lost their shirt? Do they just ship me enough Brent Crude to sell it for $1.5M at spot prices? What if they were lying and they can't deliver ?
Yes, there are physically settled and cash settled futures. The futures price converges to the settlement price (tied to spot) as the expiration date approaches.
Futures are marked to market every day and gains and losses are paid out daily in cash. So in the example you gave, you would have received $500K profit between when you bought the futures at $1M and when they expire at $1.5M. For cash settled futures that's the end of the story - a final small adjustment happens, in cash, your position disappears, and that's it.
For physically settled futures, the story is the same, but on the day of delivery, you (as the buyer) would be required to pay $1.5M cash to receive the oil, and whoever held the other side of the contract (as the seller) would be required to deliver it to you at some named location, typically a storage facility.
Who loses the $500K? The seller (who was short the contract) has been paying the losses daily as the price moved against them. The payments are handled through the exchange/clearinghouse and their brokerage.
What if they can't deliver? The exchange steps in and provides guarantees.
If I bought $1M of Brent Crude Futures settled by delivery so I can make fuel for cars and you "step in and provide guarantees" rather than delivering I cannot refine "guarantees" into fuel for cars.
Ordinarily I'm sure such "guarantees" are enough, maybe for 10% above spot price we can get enough oil to my refinery to make that fuel and the exchange is able to cover this cost from their operating budget. But when the deliveries don't even resemble sold futures that stops working, that oil isn't available at any price and so those "guarantees" become worthless.
This is the other end of the problem the ETFs had last time. The ETFs couldn't go negative because an ETF is just paper so if my ETF had a notionally negative value I don't pay I just tear up the ETF instead whereas the notionally negative value of an oil delivery was like yeah, this is nasty toxic black sludge, you're going to need to put that somewhere.
The other end is that ETFs aren't actually oil, if I have an ETF but I need diesel that's not a thing you can make from the ETF. Even if my ETF tracked the price perfectly, I still don't have diesel, I need to find somebody who wants to sell the diesel and maybe if supplies are tight that's not one of the options.
See the spread when Russia attacked Ukraine went up to 100
p.s. TIL contango is opposite of backwardation