That would probably be a pretty classic contract law question. Generally speaking, a modification or amendment of a contract is only valid if it is agreed to by both parties. You would want to show that both parties agree by a writing, such as both parties initialing the modification or amendment or explicitly stating that the modification or amendment is agreed to and incorporated into the contract. Something like you described would probably be treated as a unilateral mistake and would not necessarily render the contract void. However, there are probably several linear feet of appellate court opinions in that jurisdiction on how to treat mistakes in contract, and it would depend entirely on the statutory/case law of a given jurisdiction as to how this matter would be handled. A lot of time would be devoted into looking into your intent in making that change and did you have 'unclean hands' in doing so.
First off, I'll state that IANAL. And this story is a little short on details, since it's at least 10 years old and reproduced by memory.
But I remember reading that a company had sent some sort of tech agreement or NDA out to a new hire for his signature; he didn't like the terms, so he changed them, printed out a new form, signed it, made a copy for himself, and sent the signed version back to the employer,
who accepted it without comment and hired him.
Fast forward a couple of years . . . he separated from the company, the company tried to enforce what they THOUGHT was his signed agreement, but when it came to court and they were ordered to produce THEIR copy . . . with his signature, it turned out to include things they didn't know were in there. In fact, IIRC, they didn't know the form was different until the guy's lawyer had THEIR guy read a few specific lines in front of the judge.
The separated employee won.
As for technical agreements - the one I signed (unaltered) put some restrictions on working for a competitor for 2 years without my employer's permission, but if they kept me from working, they'd have to continue my salary for that period.