After Kenneth loses his pants entering Nancy's house through the dog door, Jack hands them back to Kenneth and he puts them on. There is no belt in the scene, but when Kenneth removes the pants later to leave through the dog door, he is wearing a belt.
When Jack and Kenneth are listening to the voicemail, it says the dates are Thursday, December 30th and Friday, January 1st. Aside from the fact that December 30th was a Wednesday in 2009 (when the message would have been left), December has 31 days, so one was missed as there are no days between Thursday and Friday.
When Kenneth gives advice to Jack after erasing the message, he mistakenly refers to Nancy (Julianne Moore's character) as "Mrs Doherty" instead of Mrs Donovan. However, this is not a goof. Her married name is Doherty, and her maiden name is Donovan as stated in Season 4 Episode 16, "Lee Marvin vs. Derek Jeter." Jack refers to her as Nancy Donovan because he knew her by her maiden name in high school.