At video shop, there is a poster from Futurama on the wall and a video with title Family Guy (though Family Guy and Futurama wouldn't see a home video release [in the form of DVDs] until the mid-to-late 2000s due to cancellation. The DVD sales and constant reruns on cable TV is what revived both shows).
On the airplane, when Marge describes First Class as "feeling like Princess Grace and Princess Di smashed together", it is a dark reference to their deaths -both died in car crashes.
When Homer and Marge finally 'ditch' Bart and Lisa, the plate cover is small at first, but when Bart and Lisa emerge from beneath the plate cover it has obviously gotten bigger.
When Bart, Lisa and Abe go to Miami, Maggie is absent, as she is throughout the whole episode. Her absence is suspicious since Abe would surely babysit Maggie as well as Bart and Lisa. However, this may not be a goof as Patty and Selma could be babysitting Maggie, though this isn't likely. -Maggie can be seen sitting on the couch when they watch the tornado on TV.
When Homer and Marge are packing and Homer goes to strangle Bart through the wall, he punches through the wall next to their bed which has been shown to be an outside wall. The kids rooms are through the opposite side wall.
Boarding passes are checked at the gate and on the aircraft, therefore Homer and Marge would have been stopped when they got to the aircraft, even though they were allowed through at the gate.
Homer and Marge's passports weren't checked at the gate.
Homer and Marge's hand luggage was too large and therefore would have to go in the hold.
Homer, Marge, Bart, Lisa and Grampa couldn't have got flights at such short notice.
Homer and Marge could not have boarded a flight they did not have boarding passes for.
Homer and Marge would have had nowhere to stay when they got to Florida.
There would not have been two flights to the same place within such a short time period.
It is fraud to use someone else's credit/debit card without their permission.
Homer and Marge's luggage would be on the flight to Ohio until they were off-loaded due to not showing up, meaning that they would have no change of clothes.
Also they are seen to be wearing swimwear, despite the fact that they didn't have any clothing other than the clothes they traveled in. They could have bought the swimwear from a shop.