From Longman Dictionary of Contemporary Englishby accidentby accidentCHANCE/BY CHANCEin a way that is not planned or intended OPP on purpose, deliberately I met her quite by accident (=completely by accident). The discovery was made almost by accident. The pilot, whether by accident or design (=whether it was planned or not planned), made the plane do a sharp turn.Grammar ✗ Don’t say: by an accident → accident
Examples from the Corpus
by accident• Fleming discovered penicillin almost by accident.• The trigger of the gun is locked so that it cannot be fired by accident.• He brought over some of my mail that was delivered to his house by accident.• Lombardi heard about their plan quite by accident.• The fire started by accident.• We ended up by accident on the wrong train and had to ride all the way to Montreal. 