Legal Question in Real Estate Law in California

I don't know, but my hunch is that a simple mistake like forgetting to check a box would not invalidate the counteroffer if everyone knew and admits that it was a mistake, and not the seller's intention. The law of contracts has a strong preference for enforcing the parties' intent, in preference to admitted mistakes in filling out forms. It may depend upon the timing of events. If someone notices they made a mistake and point it out to the other party promptly, the effect of the mistake is even more likely to be minimal.

No, you original offer is only enforceable when it is accepted. The fact that the seller made a counteroffer means your offer was rejected, as a matter of law.

If the documentation was handled the way it usually is, Roach is dead wrong. What I think you mean happened is that the sellers signed the acceptance on the original contract form, and submitted a counter offer form, but didn't check the box on the original contract form that says "subject to attached counter offer." Technically that means your offer was accepted as written and the counter is invalid. As Mr. Whipple points out, however, submitting the acceptance with a counter, but not checking the box, created an ambiguity that does noto void the contract and that would be interpreted by the clear intent of the parties that the counter should be part of the deal. If you then accepted the counter, that is further clear evidence that the acceptance was subject to the counter.