When Mona and Homer are on the couch, Homer's pie plate suddenly disappears in the final shot of the scene.
When the Simpsons come home, they find the door open. The Simpsons then open the door while entering the house.
When Marge says none of the kids remember any of the nice things they do for them, there is no garbage can around the Simpsons at all. In the next shots, there is a garbage can next to Bart for the kids to throw their sweaters into.
When the Simpsons are walking to Stuff-N-Hug (where Maggie wanted to go), Homer is not with the other Simpsons where he was previously seen walking to Stuff-N-Hug with them. When the Simpsons enter Stuff-N-Hug, Homer is back with the rest of the Simpsons.
When Homer and Mona are talking on the couch, Homer puts the pie on the counter and has only taken a bite out of it. When Homer says he doesn't want the pie. Mona is holding the empty plate, showing Homer ate the whole thing. After Homer put the pie aside, he was not seen eating any more of the pie. Plus, Mona was never seen getting up off the couch to pick up Homer's (suddenly) empty plate.
Marge burns a bag made from hemp to make guards high. Hemp fiber is made from the plants bast, whereas marijuana is made from the seeds. Burning a hemp bag would not make you high.
When Homer is destroying the "Record-Your-Voice" Critters, he uses a stuffed unicorn to destroy some of the critters by using the horn to stab the critters. The horn should not have been able to stab the critters because it should also be stuffed fabric material and would be way too soft to put any damage into anything.