Procrastination / I'm With Stupid - S2-E19
Continuity mistake: In 'Procrastination' when SpongeBob takes the stack of paper out of the drawer, the paper is the only thing in the drawer. In the next shot, SpongeBob pulls a pencil out of the drawer.
The Graveyard Shift / Krusty Love - S3-E13
Continuity mistake: In 'The Graveyard Shift' Squidward explains that there are three signs concerning the arrival of the Hash-Slinging Slasher. He then holds up three hands. In the next shot, he is only holding up one hand.
Sold!/Lame and Fortune - S9-E21
Continuity mistake: In 'Lame and Fortune', a big box of fortune cookies drops in SpongeBob's backyard. When Gary goes to read his fortune, he leaves a slime trail. However, the slime trail disappears and reappears between shots.
Born Again Krabs (aka A Penny Saved) / Safety Freak (aka I Had An Accident) - S3-E17
Continuity mistake: In "Born Again Krabs", the doctor taking care of Mr. Krabs drops his clipboard and runs. When the Flying Dutchman appears in the next shot, the clipboard is missing.
Born Again Krabs (aka A Penny Saved) / Safety Freak (aka I Had An Accident) - S3-E17
Continuity mistake: In "Born Again Krabs", Mr. Krabs drops the coins that the Flying Dutchman gave to him on the floor. In a few shots, when the Flying Dutchman comes back, the coins have vanished.
A Pal for Gary/Keep Bikini Bottom Beautiful - S7-E14
Continuity mistake: In "A Pal For Gary", Spongebob opens the door to his library, causing it to fall off its hinges and drop about 5 feet, out of frame. In the same shot, the camera zooms out to reveal that the door is missing.
New Digs / Krabs à la Mode - S5-E6
Continuity mistake: In "Krabs à la Mode", Spongebob is wearing miniature mittens on his eyelashes while walking to the Krusty Krab. They disappear after a few shots.
Squidward's School for Grown Ups - S8-E18
Continuity mistake: Spongebob uses a razor to try and shave off Patrick's beard, which falls onto the ground. In two shots, the razor disappears.
Best Day Ever/The Gift of Gum - S4-E20
Continuity mistake: When SpongeBob tries to steal the gum at night, he's dressed as a bandit, but then the next day when Sandy finds him, he's wearing his normal clothing. He was stuck in the gum all night so there's no way he would have changed.
Continuity mistake: In "The Krusty Drive-through", Mr. Krabs creates a drive-through. However, an updated one can be seen in older episodes.
Karen 2.0/InSPONGEiac - S8-E28
Continuity mistake: In "InSPONGEiac", Spongebob drinks from a glass and drops it on the floor. In two shots, the glass has disappeared.
Continuity mistake: In "Once Bitten", there is a keyhole on one of the doors to the Krusty Krab. Several shots later, the keyhole disappears.
Nature Pants / Opposite Day - S1-E9
Continuity mistake: In "Opposite Day", Spongebob pushes Squidward through the sand, leaving a pile of sand. Two shots later, they are now on Spongebob's sidewalk and the sand pile has disappeared.
Nature Pants / Opposite Day - S1-E9
Continuity mistake: In most of the show, Squidward's house only has windows on the front of his house. But, in "Opposite Day", he has windows on the "ears" of his house, which don't appear throughout the show.
Nature Pants / Opposite Day - S1-E9
Continuity mistake: In "Nature Pants", Spongebob comes back home, where his friends have thrown a "Welcome Back" party for him. When he enters, two balloons float behind him. In the next shot, they disappear.
Dying For Pie / Imitation Krabs - S2-E6
Continuity mistake: In "Dying For Pie", Mr. Krabs takes a piece of the pie, leaving a bit of the pie missing. Later that day, Spongebob pulls the same pie out of his pocket, but the pie is fully together with no pieces removed.
Enchanted Tiki Dreams - S7-E23
Continuity mistake: Spongebob goes to set a canvas on the ground. On the spot where he is putting the canvas down, there are three flowers on the ground. In the next shot, after he set the canvas down, the flowers have disappeared.
Krusty Towers/Mrs. Puff, You're Fired - S4-E9
Continuity mistake: When the fish is at the front desk of the Krusty Towers, there is a silver bell on the desk. When Patrick arrives, there is no bell. Then Mr. Krabs puts Squidward in his place, and the bell is back, but has moved to the opposite side of the desk.
Bubblestand / Ripped Pants - S1-E1
Continuity mistake: In "Ripped Pants", we see 3 other people at the square lifting weights besides Sandy. When Sandy lifts her weights, the crowd cheers, and we get a wide shot of the square, we see that the 3 thugs are gone.
Help Wanted / Reef Blowers / Tea at the Treedome - S1-E2
Continuity mistake: When we first see a far shot of Squidward's house, there is no potted plant on his porch. When Squidward steps out, the potted plant appears, and he rights the crooked flower. The potted plant disappears in the rest of the shots of Squidward's house.
Answer: I believe this is the Season 6 episode "Spongicus".