When Tommy says to Alyssa, "You were kidding?" the lid on the jar of fruit flies is sitting askew on top of the jar. When he says his next line, "Yeah, well, I'm the kind of guy that actually does things," the lid is screwed tightly onto the jar.
When Sally speaks to Don about appearing in the magazine, her hair changes between shots.
When Harry walks naked behind Don in the kitchen you can see the band of the string on the side of his hip.