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.