The penalties may vary from state to state, but generally the penalty will be felony fraud and impersonation. This will most likely result in jail if not prison time. Official documents are not something to mess with and the courts do not treat it with ease.
The court issues a summons. While it may be later determined that the individual is not relevant to the case, the summons would not be considered "illegal. "
If anyone in the court system finds out that you have improperly sent a summons to another, the court may hold you in contempt of court. In addition, you may be liable for attorney's fees and costs.
If an illegal court summons is sent it can be meet with jail time or a very large fine directed toward the person who sent the illegal court summons in the first place.