Have You Seen This Snail? - S4-E3
Continuity mistake: In "Where's Gary?" the length of the paddle ball string that SpongeBob uses seems to vary in certain shots. While the length is somewhat subjective one shot that is more conclusive is when Patrick finds Spongebob lying on the floor. The string on the paddle ball is fairly short. When the two get up to get a glass of milk, the string is now a lot longer.
Have You Seen This Snail? - S4-E3
Other mistake: When Gary comes back at the end, when Gary meows and Spongebob says, "Yeah, like that," the tears in his eyes disappear for about half a second. (00:22:50)
Have You Seen This Snail? - S4-E3
Continuity mistake: Have You Seen This Snail: After the customer asks for a refund it cuts to SpongeBob crying in the kitchen, and when Krabs walks in the door opens at the left side with no knob, but when Krabs tells SpongeBob that a problem can be solved with a little hard work, that door now opens at the right side with a door knob.
Have You Seen This Snail? - S4-E3
Revealing mistake: After SpongeBob tells Patrick, "I can't find Gary," text that reads "Rough Draft Korea" briefly appears on screen during the bubble transition. (00:08:24)
Answer: I believe this is the Season 6 episode "Spongicus".