Factual error: While Meg Ryan is typing her letter to "Sleepless and Son," Rosie O'Donnell is sitting on the couch watching "An Affair to Remember." Any true AAtR fan can easily notice that the film's editing has obviously been altered by the Sleepless director - the scene with Deborah Kerr and Cary Grant leaning on the boat railing actually takes place towards the beginning of the movie, while the scene of them looking up at the Statue of Liberty takes place about four days later. However, in Sleepless, the order of the AAtR scenes shown on Meg's TV are incorrect - Kerr and Grant are leaning on the railing as the sequence starts, then jumps WAY ahead to them staring up at the Statue of Liberty (which then would have been about four days later), and ends with them standing next to the boat's railing again (four days before).
Factual error: Sam and Jonah go by motorboat from their houseboat that is moored on Lake Union to Alki Beach in West Seattle. Annie follows in her rental car, watching them from her vehicle "the entire way." This is impossible. To get from their houseboat by boat to Alki Beach (near where I live) involves going over the lake, through the Ballard locks, and across Elliot Bay (Puget Sound). The only road into West Seattle and to Alki Beach from the city does not follow their water route, and it is further south and goes over the West Seattle bridge - some miles from the lake and too far away for her to be able to see them on the water. The boat ride to the beach would also take over an hour with the wait to go through the locks, though they appear to reach it much sooner.
Factual error: Jonah points to the LP record and tells Sam that "Paul is dead" can be heard when the record is played backwards. He is not holding a Beatles' record, however, as can be seen from the album's label. In the US, the Beatles recorded under Capitol Records, and their labels had a multicolored circular band printed on both sides. This label is red.