At the beginning of the episode when the gang names off restaurants to go to, the three they suggest are named "Hurley", "Michael", and "Charlie". All three are names of characters from Lost, which also was in its fourth season in 2008.
The first time Robin's dad is shown and, while played here by Eric Braeden, he's played by Ray Wise throughout the rest of the series.
Robin's full name is Robin Charles Scherbatsky Jr.
During Barney's visit to his prison girlfriend, the two inmates start fighting and throw each other into the concrete wall behind them. However, the wall clearly flexes like a set piece, not a concrete wall.
In the cab, Ted's seat belt is crooked.