This paper describes a website created to test the hypothesis that nobody bothers to read the TOS, and it contained a privacy clause saying that "we may share everything, and a clause that
by agreeing to these Terms of Service, and in exchange for service,
all users of this site agree to immediately assign their first-born
child to NameDrop, Inc. If the user does not yet have children, this
agreement will be enforceable until the year 2050. All individuals
assigned to NameDrop automatically become the property of NameDrop,
Inc. No exceptions.
Since this was not a real webpage, there was no attempt to enforce.
TOS is just another kind of contract, so the question is which kinds of contract conditions are enforceable and which kinds are not. It depends on whose laws you are operating under, which explains the use of expressions like "void where prohibited by law". There are innumerable conditions that could be included in a contract which are unenforceable. A contract requiring a person to commit a crime is unenforceable. Many jurisdictions have laws prohibiting a person from waiving certain rights, for example you cannot sign away your right to be represented by legal counsel in a dispute. There are related "unfair / deceptive" practices laws, which might include meta-conditions that any clause disclaiming liability must be prominently displayed.
These are terms that a reasonable person would clearly know in advance are illegal and unenforceable.
There is also a concept of "unconscionability", a finding that a certain condition favors the business to the point of "shocking the conscience", which may take a deeper legal analysis to evaluate. A requirement to litigate small disputes in California might be deemed unconscionable for a customer in an East Coast state, but it might not be. A typical characterization of an unconscionable contract is one that "leaves one party with no real, meaningful choice and is unreasonably advantageous to the other party", especially when due to asymmetrical negotiation power. For example, Ellis v. McKinnon , 18 Cal. App. 4th 1796 (employment contract for a salesman, commissions were forfeit if company had not received payment from the customer by the termination of employment). The main case law in the US in this area is Williams v. Walker-Thomas Furniture, 350 F.2d 445, where a customer bought furniture on credit, with a clause allowing repossession of all of the furniture in case of default by the customer. The doctrine is encoded in UCC 2-302. However, "unconscionable" does not mean "I don't like it". In lieu of statutory price controls, a customer probably cannot avoid paying an agreed on exorbitant price for a product ($100 for a dozen rolls of toilet paper). Here is a bit more legal analysis of unconscionability, which focuses on three factors: one-sidedness, oppressiveness, and likeliness to result in unfair surprise.