In episode 910 entitled 'Follow that egg' Stan makes a phonecall to Kyle (about 5 and a half minutes into the episode) to check up on him and Wendy. In the shots where Stan is talking you can clearly see a glass of water on the table next to his elbow but when he hangs up the phone the glass has disappeared. He obviously didn't drink it or move it because he didn't have time. By his sudden change in expression it looks like they cut some shots out and forgot about the glass.