At the bar Newton adds lines to the drawing on the napkin. When Gwen looks at it the next morning they are gone.
When the mom gives Gwen the card with the money, Gwen closes the card twice between shots.
When Gwen is in the house discussing "what happened in Maui" with Becky, her sweater sleeves are pushed up by her elbows. After she finishes the story she leaves the house they are now down around her wrists.
When Davis is describing the town of Dobbs Mill to Gwen on the night they first meet, he says the town is especially beautiful this time of year when the leaves are changing, which happens in Massachusetts in autumn. But when Gwen first gets to town and is seen walking around looking for the house, the trees are green and there are bright flowers growing, so it is clearly spring or summer.
In most areas squatting, essentially what Gwen is doing in Davis' house, is illegal. Davis could have easily pressed charges and had her forcibly removed instead of going along with her ruse.
About 37 mins in, when Steve Martin falls over the couch and hits his feet on the table, Goldie Hawn briefly grimaces and looks at the crew possibly expecting a 'cut' before the scene continues as scripted.