Common error is where many have fallen who have interest in knowing a fact of which they are however ignorant. It is a maxim in law ( droit ) that error communis facit jus  [1], that is to say that it exempts one who has fallen, as have the rest of us. In the books of Justinian there are two remarkable examples of the effect that common error produces.

One is in the famous law barbarius Philippus ff. de officio praetorum  [2] . It concerns a case in which a slave has attained the office of praetor; the law decides that everything he has done is valuable.

The other is the law, si quis in ff. de senatusc. maced. which decides that if a man is tried with a son of the family, who publicly passes as father ( père de famille ), this son will not be able to plea against him by Macedonian benefice, quia publicè .... sic agebat, sic contrahebat . [3]

Notes

1. "Common error makes right" or "common error makes law". For a nice statement of how this phrase first appears in the middle ages, tagged alongside the ancient Roman legal texts from which it ultimately stems, see Ciacli, Petru and Ioana Mihnea. "Error communis facit jus. Origins. The Roman Law, the Middle Ages and the Old Romanian Law". 2nd International Conference on Humanities, Historical and Social Sciences IPEDR vol.17, 2011. The authors remark, "The work of the glossators was edited in 1250 by Accurius, Glossa Ordinaria . The fact that the glossators had written down alongside Ulpian’s text, referring to the slave Barbarius Philippus the following text, Notatur hic quod circa factum error communis facit jus , is remarkable."

2. According to the Dictionary of Greek and Roman Antiquities edited William Smith 1870, p. 461, "Barbarius Phillipus" is a variation of the name "Barbatius Phillipus", belonging to an ancient Roman freedman, Phillip Barbat. Once a runaway slave, he won the favor of Julius Caeser, was freed, and became praetor. Justinian remarks of Barbat's assumption of this office as a legal precedent for e rreur commune , cf. Krueger, Paul, Theodor Mommsen, Rudolf Schöll, and Wilhelm Kroll. Corpus iuris civilis. Dublino, Turici: Apud Weidmannos, 1963 , §1.14.3: " Barbarius philippus cum servus fugitivus esset, romae praeturam petiit et praetor designatus est. sed nihil ei servitutem obstetisse ait pomponius, quasi praetor non fuerit: atquin verum est praetura eum functum. et tamen videamus: si servus quamdiu latuit, dignitate praetoria functus sit, quid dicemus? quae edixit, quae decrevit, nullius fore momenti? an fore propter utilitatem eorum, qui apud eum egerunt vel lege vel quo alio iure? et verum puto nihil eorum reprobari: hoc enim humanius est: cum etiam potuit populus romanus servo decernere hanc potestatem, sed et si scisset servum esse, liberum effecisset. quod ius multo magis in imperatore observandum est." [Were Phillip Barbat a freed slave, he shall be designated praetor and may ask for a lesser praetorship, because the emperor shall keep nothing of his servitude, were he not praetor, and so, the praetorship is his true function, and we see then that: if the slave has hidden himself for a while, his praetorship is performed in dignity, and then what do we say? How may anything so announced or described not have its own occassion? And because these advantages shall exist, who shall follow them as closely as some law or other right? And so I do nothing to reject their truth. Indeed, this is human, for could I watch over the Roman population and likewise grant this power? Since and if the slave is known publicly, he shall make himself free, which is an extremely great justice to be observed in the empire.]

3. Krueger, Paul, Theodor Mommsen, Rudolf Schöll, and Wilhelm Kroll. Corpus iuris civilis. Dublino, Turici: Apud Weidmannos, 1963 . §14.6.3 "Ulpianus 29 ad ed. Si quis patrem familias esse credidit non vana simplicitate deceptus nec iuris ignorantia, sed quia publice pater familias plerisque videbatur, sic agebat, sic contrahebat, sic muneribus fungebatur, cessabit senatus consultum." [After Ulpanius. Since whoever makes a loan to a head of household is not deceived by vain simplicity nor ignorant of the law, but because the head of household and a good many are observed publicly, he thus has acted and entered into contract and has performed service, and shall then be free of the decision of the senate.]