Last August my difficulty with my U.S. passport began in Berlin. After a full day of museum visits, sidewalk cafes, sightseeing and people-watching in East Berlin my passport and visa were closely examined by several officials. One finally explained that the small cut in the laminate alongside my passport photograph could indicate that my photo had been substituted for another.
Upon returning home I visited the Passport Agency and complained strongly. I was told that a number of passports had had similar problems due to an inferior laminating process. Recently issued passports apparently use a more durable laminate which does not easily split. As Mr. Speers pointed out, it pays to check your documents before you travel.