A quick definition of plus petitio:

Plus petitio is a Latin term that means "overclaim" or "claiming too much." In Roman law, it refers to the mistake of claiming more than what is due in one's pleadings. This was not allowed under classical law, but under cognitio extraordinaria, a claimant could continue the action but could be liable for treble damages to any person injured by the overstated claim. Plus petitio can occur in different ways, such as claiming a bigger amount than what is due, claiming before the payment is due, claiming at a different place than where the payment had to be performed, or claiming a certain thing although the debtor had the right to choose between two or more things. After the abolition of the formula-regime, plus petitio lost its actuality, and in Justinian's law, the plaintiff lost the case only if he maliciously persisted during the whole trial in his overclaim.