In the DS9 Episode "Trials and Tribble-ations", Sisko is having a conversation with two members of Temporal Investigations. The conversation goes like this:
DULMUR: Be specific, Captain. Which Enterprise? There've been five.
SISKO: This was the first Enterprise. Constitution class.
DULMUR: His ship.
LUCSLY: James T Kirk.
SISKO: The one and only.
But according to Star Trek canon, the first Enterprise was commanded by Jonathan Archer, as shown in the prequel show Star Trek: Enterprise, not by James T. Kirk.
Most likely, this is just one of those mishaps caused by having a prequel come out after the original show was released, but has any writer or cast member ever tried to explain or justify this line?