Despite Cameron's apparent obsession with historical accuracy, right down to the smallest of details, there were still a few errors. At one point Jack tells Rose that he went fishing on Lake Wissota in Wisconsin, which is impossible considering the lake wasn't formed until five years after the Titanic sank due to the construction of a hydroelectric dam.
This particular inaccuracy was one of several inconsistencies that led to a popular fan theory that Jack is actually a time traveler.