When Jack is showing Seinfeld the Milf Island promo, the announcer says that there are "25 sexy moms", but the screen displays "20 moms."
When Liz and Jack walk into Jack's new office, you can see what looks to be a crew member's hand holding something on the left of the screen.
Liz and Jenna act like they haven't seen each other all summer, but they are best friends, so they would likely have seen each other at least once during the summer, even if Liz didn't go to Jenna's musical.