It's not illegal to accidentally write 1 or 2 checks with insufficient funds if it was just an accident that happened on occasion.
But if you continue to write checks with insufficient funds then it can become illegal as it could be seen as purposely writing checks with insufficient funds which can be looked at as check fraud which can get you into jail for a year or more.
If the check writing of insufficient funds doesn't happen too often and was just a mistake then nothing will usually happen except you having to pay a returned check fee to the business you wrote the check at and pay a overdraft fee to the bank plus the money the check was written out for.