Continuity mistake: After the kids have left the party, Pluto goes to the table and on the table there is a big blue plate of where the cake was. At this point you can see there is nothing next to the plate, but in the next shot a small blue plate has appeared.
Continuity mistake: When Pluto blows out the candles on his birthday cake, you can see the words "Happy Birthday Pluto" on the cake, but when the kids take slices of the cake in the next shot, the words have disappeared.
Continuity mistake: When Pluto is dizzy after being spun around the young mouse, he walks into a tree and you can see there is nothing behind the tree, but in the next shot a fence has suddenly appeared.
Continuity mistake: At the start of the cartoon, you can see that there are six small trees around the bottom of Pluto's birthday cake. But after Pluto has had his bath, there are now four trees, and after Mickey tells him to wait for his guests, there are now five.
Continuity mistake: When Pluto takes the name plate off his kennel, he puts it into a bowl and takes it to the table. At this point, you can see that there is a dish and a name card saying "Jerry" next to him, but a few seconds later Jerry's name card and the plate have disappeared.
Continuity mistake: Before Mickey Mouse puts Pluto's birthday cake on the table you can see it is completely clear, but when Pluto is sitting down and the cake is in front of him, a saucer has suddenly appeared.