No, it is not possible to miss an ulcer if active on endoscopy. However, it is possible if an ulcer is in between folds and the stomach does not open up during the procedure. This can happen in inexperienced performers as well as with tiny ulcers. Statistically it is possible that the diagnosis is not 100% but a recently healed ulcer can give rise to symptoms but late endoscopy may not show any, as ulcers heal very quickly. I must qualify and say that I am talking about peptic ulcers only and not cancerous ulcers.