François talks dirty to Sheeni in the boarding school ("I want to tickle your bellybutton from the inside", etc.) Director Miguel Arteta revealed on the commentary that all those lines came from researching internet porn sites.
When Nick is stealing the BMW, his father kicks the window with his foot leaving two footprints on it. After a couple of scenes the marks have moved upwards from their first location.
Two similar E46 BMW 3 series vehicles are clearly visible: first a 1999-2001 and after 2002-2005. You can notice the two similar vehicles with Nick Twisp (Michael Cera) faking his death.