Errāre humānum est is a quotation from Seneca, a Roman philosopher of the first century AD. It means to err is human, but there is a second half to this famous line, i.e. to persist in error is diabolical. (Sed perseverare diabolicum.)
Seneca strikes an ambiguous figure in Roman history, well known both as a strict stoic philosopher and as the tutor and speech writer for the brutal emperor Nero.
In the end, suspected of treason and conspiracy, Seneca was forced by Nero to commit suicide. Seneca seems to have turned his own death into both a dramatic performance and a final lesson in the philosophy he espoused.As recommended reading, two works come to mind: For a selection of letters to sketch the man and his life, read Letters from a Stoic (Penguin Classics). If you would like a sample of his philosophical writings, a good bedside read is On the Shortness of Life (Penguin Great Ideas). If you would like a modern interpretation of Seneca's life and work, I recommend Dying Every Day: Seneca at the Court of Nero, by James Romm. Errare humanum est, sed perseverare diabolicum.