In "The Show Where Sam Shows Up" Sam mentions that Woody and Kelly had a baby boy, however neither Frasier nor Woody make any mention of his son in this episode, just his daughters.
Daphne starts singing at 15:20, whilst holding a glass of wine. Then we get a shot of Martin looking at her. The moment the shot returns to Daphne, we see her glass being fuller than it was a split second ago, even though a few moments back the bottle was completely emptied into the glass.
At Nervosa, when Frasier is begging Niles to join him and Woody, the barista brings Niles' triple espresso to go and plops it down on the counter. It's obviously just a prop because the clunk of the cup signals it's completely empty.
When Frasier (Kelsey Grammer) gives the tourist guide to Woody (Woody Harrelson) and Woody says "look at that skyline" in reference to a picture in the book and Frasier tells him to turn around we see that there is no picture in the book.
When the barista places Niles' triple espresso to go down on the counter, the hollow sound clearly reveals the cup to be empty.
Niles orders a triple espresso to go in Café Nervosa, but the barista hands him a much larger cup than would be normal for three espresso shots.
At the start of the show it is revealed that Woody (Woody Harrelson) was lost while driving around the airport, then when he is going back to Boston, he says his cab is waiting downstairs. He had his car so he wouldn't be taking a cab. However, Woody is lying about going to the airport, and uses the alleged cab waiting to create a time restraint.
Frasier states that Woody had been working at Cheers for 15 years. Woody's first appearance on Cheers was in September of 1985. The episode of Frasier aired only about 13.5 years later, in February of 1999.