In the police car, one of the ladies draws a thin mustache above Jack Lemmon's right upper lip. Next shot it is gone.
When Nestor changes into his old uniform to elude detection, he still has the ribbon he was awarded for saving a child's life even though he had previously said it was taken from him when he was kicked off the police force.
When Nestor and Irma first meet, Nestor says he received a medal for performing 'mouth-to-mouth respiration' on a drowning child. This is another term for 'mouth-to-mouth resuscitation'.
When Irma goes to throw Nestor out after he climbs in the window she lays out "what ya came with" she lays out his policeman's uniform. He was dismissed as a policeman *before* they became involved, BUT he came to Irma with his uniform in his suitcase (it was almost everything he had in that suitcase).
Nestor opens his baggage after his first night at Irma's place, and shows it.
Music in a large Catholic cathedral such as that in the wedding scene is always played on the magnificent pipe organ, but the organ heard in this wedding scene emanates from a cheap model of electronic home organ.
The shadow of the boom can be seen on the brown wall, at the right of the screen, just after Nestor shows up in Irma's apartment following his jailbreak. It shows up behind Lefevre just after Irma's sarcasm that Nestor can be found in jail.