About 15 minutes in Judy and Laura are talking in the kitchen. When Judy get's up from the table the napkin falls from Judy's right knee. In the next wide shot you can see under the table and the napkin is now on Judy's left knee.
Near the end, Carl says "I just can't seem to get warm" and Harriette says "How about this?" as she throws Carl back in the couch. But when the camera cuts in for a closer look, a picture behind the couch is wavering even though it was upright when they started kissing.
Eddie bursts through the front door, but doesn't close it behind him. Moments later, the door is closed when Carl enters.
The show is set in Chicago and Lake Winnamuck is shown with large mountains in the background. They'd have to drive hundreds of miles to find mountains like those. They're obviously in front of a fake backdrop.
When Carl and Eddie stop walking on the ice,just below their waists you can see where the wall met with the ground that shows the mountain background for the lake.
They're supposed to be on a lake in the middle of nowhere, but the studio lights are frequently reflected in Urkel's glasses.
When Rachel introduces Richie in his penguin costume she says, "Direct from the Arctic..." Penguins are an Antarctic species.