I guess it isn’t illegal, unless you think doing something stupid should be illegal.
But I can see how this can happen. You are presented with the perfect business opportunity and are pressured to enter into a contract. You know that you should probably wait until your LLC has been formed, but you cannot wait until "FileYourLLC.com" actually gets around to doing it (you opted for the "basic" LLC formation package which indicated 2-3 weeks processing time).
So you figure, why not sign in the name of the LLC, nobody is going to ask later. Bad idea. If you sign for your LLC despite the fact that the LLC has not been formed, you are likely personally liable on whatever obligation you signed for. You may or may not be able to get the other party to the contract release you once the LLC has been formed.
So, if at all possible, wait until your LLC has been formed by filing the articles of organization with the Department of State. If you are really in a hurry and are ready to pay extra filing fees for expedited handling, you can form an LLC within 2 hours, so there is really no reason to risk personal liability on any contracts entered into before the LLC came into existence.
**This post is for informational purposes only and does not constitute legal advice**