When SpongeBob goes to the factory of Gary's treats, and the worker tells them it's closed, and spells out the word out as "C-S-L-O-S-Z-E-D" which it's really spelled "C-L-O-S-E-D."
In the photo of SpongeBob getting Gary from the pound, he looks the same age as he is now, while in "Have You Seen This Snail?", he was a child when he had Gary.
When SpongeBob sees Gary on top of one of the tables, Gary's swirl and spot colors are reversed.