First time that "The Bro Code" is mentioned.
When Barney and Robin talk about Battleship at the bar, Barney mentions that he cheats by stacking up the boats. Later, when they play a game, you can see on Barney's board that his boats are all in one stack.
First time we learn the Ted, Marshall & Lily's apartment address (4'40) "75th street and Amsterdam" (Upper West Side, NY).
While Lily and Marshall are waiting in the bathroom, Lily comments she had an entire Big Gulp while she and Marshall watched a Quantum Leap marathon. Marshall responds by saying "Oh, Boy". In Quantum Leap, Dr. Sam Beckett would often exclaim "Oh, Boy" when he leaped into a new person.
Lily and Marshall in 1x14 when they are stuck in the bathroom say that they've never peed in front of each other. However, in 1x06, you see Marshall going with Lily into the bathroom to help her go pee because her parrot costume is "elaborate".