I wouldn't describe it as game breaking but I think you can't get the rest of the bodies contracts since I'm pretty sure even if the contract expires it just gives you it again so you can't get any other Kerbin World Firsts contacts for that body.
Edit: I tried accepting the contract and then cancelling it to try and get the other missions but apparently these milestone contracts can not be denied once accepted. Accepting this contract I think also soft locks the milestone contract lines, even the ones for other bodies.