I don't want to sound like a condescending ass, but I am a lawyer. (So I guess that makes me a condescending ass . . . ? Not exactly how I meant it, but sure sounds that way, & can't really argue.)
The '900-page' contract you mention would be unenforceable, but not because someone missed that clause. In every jurisdiction in the US there are four (depending on how they're broken down) requirements for a valid contract: competent parties, legal subject matter, mutual agreement, and valid consideration.
The first is easy - you can't hold a 6-year-old or insane person to their agreements. The second means that contracts for drugs or prostitution are not enforceable. The third means that the parties agree on what the contract is. And the fourth is that there has to be an exchange of something valuable - an agreement to give someone a gift is not enforceable (in most circumstances).
Totally aside from the fraud/misrepresentation issue of trying to trick someone out of their car, the contract you're talking about would be voidable for lack of consideration: it doesn't have to be equal, but you have to show that each side intended to give up something of value to them in exchange for what the other side was giving up.
You could argue a lack of consideration when you think you're getting an unencumbered piece of software, and the other side knows you're only getting a limited license, but you will run into all sorts of problems with that. First, the sofware vendor is going to be able to point out that everyone in the industry only sells licenses, and they'll be able to give examples of mammoth organizations like Wal-Mart or the U.S. government purchasing software on a per-seat license basis. Second, they're going to argue that there would be a failure of consideration if the transaction were structured as a sale instead of a license, because then they would be giving up all sort of rights that would be valued much higher than the $50 you forked over at Best Buy.
As for mutuality, it says right there on the box what you are getting. If you believe that you are getting more than the EULA allows, in direct contradiction to what that warning says, then that's a unilateral mistake and a unilateral mistake is not going to get you very far.
Not wanting the EULA to be binding is not the same as saying that it is. For a non-tangential precedent, see Tres Jeanee, Inc. v. Brolin Retail Systems Midwest, ___ F.3d ___ (W.D.Ky. 2007), in which the court, when faced with the validity of a EULA, looked at other jurisdictions and concluded:
a survey of cases from
other jurisdictions suggests that clickwrap agreements requiring the user to assent to their terms are generally upheld in the face of allegations of insufficient notice of terms.
The Seventh Circuit has explicitly upheld EULAs, where the sole issue in the case was the validity of a 'shrink wrap' license agreement where the terms were only a notice was on the outside of the box and the terms were inside. ProCD, Inc. v. Zeidenberg, , 86 F.3d 1447 (7th Cir. 1996). The court gives a good example in that one: warranties on consumer goods. By your argument (and that of the defendant in that case) no warranty would be valid because you didn't know the terms when you purchased the product. I would highly recommend reading the case, it's fairly clearly written. I'll leave you with one last quote, from the court's discussion of the Uniform Commercial Code (which has been adopted in most, if not all, states):
A vendor, as master of the offer, may invite acceptance by conduct, and may propose limitations on the kind of conduct that constitutes acceptance. A buyer may accept by performing the acts the vendor proposes to treat as acceptance. And that is what happened. ProCD proposed a contract that a buyer would accept by using the software after having an opportunity to read the license at leisure. This Zeidenberg did. He had no choice, because the software splashed the license on the screen and would not let him proceed without indicating acceptance. So although the district judge was right to say that a contract can be, and often is, formed simply by paying the price and walking out of the store, the UCC permits contracts to be formed in other ways. ProCD proposed such a different way, and without protest Zeidenberg agreed. Ours is not a case in which a consumer opens a package to find an insert saying "you owe us an extra $10,000" and the seller files suit to collect. Any buyer finding such a demand can prevent formation of the contract by returning the package, as can any consumer who concludes that the terms of the license make the software worth less than the purchase price.