On the other hand: imagine someone putting "by agreeing to this, you owe us $1,000,000,000 - unless you opt out in writing within 90 days" halfway down the 100-page EULA of some cookie-cutter smartphone app.

It is not at all uncommon for such absurd contract terms to be unenforceable - especially in B2C contracts, although it might even be tricky for B2B clickthrough ones.

The idea being that most contracts are fairly standard, so a lot of people will just skim through them. Putting a landmine in them is obviously in bad faith, so making it enforceable would basically make it impossible to do any kind of business at all.

Nominally, Common Law, the system of law that to a first approximation is used in countries descended from the UK, has a lot of protections of that sort. You can't put "unconscionable" terms in a contract, e.g., it is simply illegal to sell yourself into total slavery in common-law derived systems. All signatories to a contract must consent, must not be under duress, the contract can not be one-sided (this doesn't mean "the contract is 'fair' from a 3rd-party point of view" but "the contract can't result in only one side giving things but the other doesn't"), and a variety of other common sense rules.

In practice, availing yourself of any of these protections is a massively uphill battle. Judges tend to presume that these common law matters are already embedded into the de facto legal system because the people writing the laws already operated under those assumptions while framing the law. Personally, I disagree and think a lot of these protections have eroded away into either nothing, or so little that it might as well be nothing, but you have a 0% chance of drawing me as a judge in your case so that won't help you much if you try.

I wish we lived in more of a "spirit of the law" world than a "letter of the law" world, where everything needs to be spelled out, but we don't. A small minority of people enjoy Rules Lawyering their way through life, insisting on trying to "gotcha" counterparties who are acting in good faith, so as a consequence, we all have to be Rules Lawyers and everything needs to be spelled out.

When the contract is purposefully obtuse and hard to understand, that should be a valid legal defense.

When it's huge, falls upon people that can't justify a lawyer, and keeps changing all the time, one shouldn't even need to claim it. It should be automatically invalid.