Hey Arnold!

When Grandpa gives Arnold the receipt, Arnold's shoes are white.

The books Arnold sits on during chess switch positions on a close-up of Arnold and the Sewer King.

When Arnold and Gerald are running from the Sewer King, the watch in Gerald's hand switches hands throughout the scenes.

When Arnold and Gerald first see the giant hole there are workmen there, but when they approach the other kids are in their place and the workmen are on the other side.

The first time we see a flier of Arnold and Abner it's on a blue background, but the one Arnold puts up (and the billboard) is on a green background. Also, Arnold is smiling differently and his hand isn't on Abner's head like it is on the other one.

Abner and Arnold run by the same group of people as they run toward each other in the end of the episode. Also, the pools of mud that Abner jumps in and the one that Arnold and Abner roll in together aren't there in the previous scene.

A picture in Arnold's room disappears after Arnold leaves his room after talking with Grandpa in "Abner Come Home".

Where did Arnold get that flashlight when he and Gerald go down into the sewers?

After Arnold gives Abner his bath and Abner goes in the mud, right before Arnold puts him back in the tub, the sound stops for a split second.