Let's say there are 3 people Q A Z . Q makes a GPL library. A uses it and licensed it under the MIT licence purposefully. Z uses A's library in a proprietary, paid-for library and distributes it with over a million downloads when Q realizes and files an infringement case against A and then Z has also infringed indirectly. However Z cannot apply GPL to his code as it would be a commercial disaster. What will Z do?
Z will either release his/her work under GPL, or (s)he will immediately cease distribution and refund all his/her affected customers for the code which they have paid to use but may no longer use. Those are the options available (edit: approaching Q for a commercial licence is, as Felix G points out below, another option, though one that I consider very unlikely to succeed).
As covered in comments above, Z didn't pay A for the code that A mistakenly made available under MIT; indeed, A may well not have the faintest idea that Z is using A's code in this way. So it is very unlikely A would have any liability.
If Z was selling a product with a clear disclaimer that the customer bought it at their own risk and Z had no idea whether the customer was entitled to use it, then I suppose it's possible that Z would be protected from an obligation to refund his/her customers. But (a) it's jurisdictionally-dependent, (b) it's also off-topic for OS.SE, and (c) why would anybody buy a product under those terms in the first place?
At the end of the day, Z has decided to make a business out of selling someone else's work. If you're going to do that, you need to bend over backwards to ensure that you have the right to do it. Z failed to ensure that, and is enjoying the consequences.