Sandy, SpongeBob and the Worm / Squid on Strike - S2-E18
Other mistake: Squid on Strike: When the destroyed Krusty Krab is first shown, the broken Krusty Krab sign has some of its letters scattered around it. There are two Ts still attached to the sign, but there is another T on the ground right next to the sign. This is impossible because there are only two Ts in "The Krusty Krab." (00:11:25)
Sandy, SpongeBob and the Worm / Squid on Strike - S2-E18
Other mistake: Squid on Strike: When Squidward looks at himself in his bathroom mirror, his buttons are on the left side of his pyjamas in the reflection. When he is shown in bed a few seconds later, the buttons are on the same side; the reflection was not reversed. (00:09:07)
Grandma's Kisses / Squidville - S2-E9
Other mistake: Grandma's Kisses: After Patrick bites his finger while eating the cookie, his shorts are not fully coloured in at the bottom of the screen, but they instantly are once Grandma kisses his finger. (00:08:37)
Grandma's Kisses / Squidville - S2-E9
Other mistake: Grandma's Kisses: After Patrick sucks all of Grandma's cookies into his mouth like a vacuum, as the cookie rolls towards SpongeBob, his lower body is missing, then it appears once he moves. (00:07:44)
Grandma's Kisses / Squidville - S2-E9
Other mistake: Grandma's Kisses: After Patrick sucks all of Grandma's cookies into his mouth like a vacuum, a cookie rolls towards SpongeBob, and as he says, "It appears to be my lunch hour," a watch appears on his left wrist mid-shot, then disappears. (00:07:49)
Grandma's Kisses / Squidville - S2-E9
Other mistake: Grandma's Kisses: When SpongeBob goes to his grandma's house with Patrick, Grandma tells him to give her a kiss, and in the next shot of SpongeBob, his right leg is missing. (00:05:13)
Dying For Pie / Imitation Krabs - S2-E6
Other mistake: Dying for Pie: When SpongeBob seems to have exploded behind the brick wall, a brick flies out of the wall. Then, when the second explosion happens, the brick reappears in the wall mid-shot, then disappears when the wall hits the floor. (00:10:20)
Dying For Pie / Imitation Krabs - S2-E6
Other mistake: Dying for Pie: Near the end, Squidward tells SpongeBob that he spent the whole day with him doing all kinds of ridiculous things because he was supposed to explode. Right when Squidward says, "explode," a small portion of his head gets cut off briefly. (00:10:52)
Survival of the Idiots / Dumped - S2-E8
Other mistake: Dumped: After Patrick starts playing tag with Gary and SpongeBob, Gary starts crawling around Patrick's body and does that up until it's dark out. SpongeBob is not wearing a watch on his left wrist, but then he looks at his watch, which suddenly appears on that wrist mid-shot. Then, right before he picks up Gary, the watch disappears. (00:14:06)
Jellyfish Hunters / The Fry Cook Games - S2-E16
Other mistake: When Patrick takes off Spongebob's right shoe to lick, his foot appears to be his left, not his right. (00:22:05)
Other mistake: At the very end, Sandy puts Wormy inside of a glass jar. Not only does she break the bubble surrounding it, there's no way for water not to enter the jar as she holds it sideways. Wormy should be killed.
Procrastination / I'm With Stupid - S2-E19
Other mistake: I'm With Stupid: When Patrick and his "parents" make fun of SpongeBob because they believe he's stupid, he gets so frustrated that he runs through the wall, leading him right outside. This makes no sense, because they are in Patrick's house and the inside of it is underground. (00:21:35)
Other mistake: At the Krusty Krab, SpongeBob tells Squidward to write a letter. Patrick then tells SpongeBob that he ripped his paper, and SpongeBob's shoes lose their colour mid-shot. (00:06:45)
Grandma's Kisses / Squidville - S2-E9
Other mistake: Grandma's Kisses: When SpongeBob is having his "mature conversation with Patrick," he tells him, "We are supposed to be adults!" In the next shot, when the camera moves over to Patrick, SpongeBob's right hand stays visible on screen; then it vanishes mid-shot. (00:06:46)
Grandma's Kisses / Squidville - S2-E9
Other mistake: Grandma's Kisses: After Grandma pulls the plug from the floor, she kisses SpongeBob and says that he can kiss his Grandma and still be an adult. In this shot, SpongeBob's legs are completely missing. His pant legs also vanish and reappear mid-shot, and even his legs appear for a frame or two, then vanish. (00:11:48)
Grandma's Kisses / Squidville - S2-E9
Other mistake: Grandma's Kisses: When SpongeBob goes to his Grandma's house with Patrick, Grandma tries to kiss SpongeBob, but SpongeBob's reflection in Grandma's glasses is not reversed; note the holes on his face. (00:05:14)
Dying For Pie / Imitation Krabs - S2-E6
Other mistake: Dying for Pie: When SpongeBob trips on the rock at the end, he says, "Oops," before he even trips on it. (00:11:47)
Mermaid Man and Barnacle Boy III / Squirrel Jokes - S2-E4
Other mistake: Squirrel Jokes: When SpongeBob looks in the mirror backstage after finishing his second act, the holes on his face are not reversed in the reflection (one of the holes also vanishes and reappears mid-shot). The second time his reflection is shown, they are reversed. (00:20:18)
Mermaid Man and Barnacle Boy III / Squirrel Jokes - S2-E4
Other mistake: Squirrel Jokes: At the end of SpongeBob's second act, as he tells the crowd, "Thank you, you've been a wonderful audience," he opens his eyes briefly, and they are coloured yellow instead of white as they should be. (00:20:14)
Mermaid Man and Barnacle Boy III / Squirrel Jokes - S2-E4
Other mistake: Squirrel Jokes: Near the end of SpongeBob's second act, he tells the crowd to give a big hand to Sandy, but to clap slowly, because she's a squirrel. Right as he says, "She's a squirrel," the holes on the bottom left of his face disappear mid-shot, then reappear. (00:20:09)
Answer: I believe this is the Season 6 episode "Spongicus".