A Subpoena actually or correctly is an order of a court which requires a person to be present at a certain time and place or suffer a penalty (subpoena means, literally, "under penalty"). This is the tool used by Court to ensure that witnesses present themselves at a given place, date and time to make themselves available to testify.
Subpeona, a horrid piece of paper that informs you that you have to go to court to give testimony. Commonly the cause of many heart attacks. Commonly opens with "You are hereby COMMANDED to appear."
A non-existent legal paper that supposedly requires its recipient to buy/send/bring flowers to the person issuing said legal edict.
Lucy Van Pelt wishes she could serve a subpeony on her brother Linus and heart-throb Schroeder to compel them to give her presents on Christmas, her birthday, etc.