Based upon script-writer Jason Segel's experience of dealing with his break-up from Linda Cardellini, as well as three other breakups (with unspecified women) besides Cardellini. Segal has stated that the 'naked breakup' did not involve Cardellini.
In the scene where Peter meets Kemo on the beach and is given a beer, when the camera is facing Peter from behind Kemo, the bar towel is missing from Kemo's left shoulder. But when the camera is facing Kemo and behind Peter, it is there again. There are 4-5 alternating shots showing this mistake.
According to the director, it's a photo montage and not Kunis' breasts.
A Note Regarding Spoilers
The following FAQ entries may contain spoilers. Only the biggest ones (if any) will be covered with spoiler tags. Spoiler tags have been used sparingly in order to make the page more readable.
What are the differences between the theatrical version and the unrated version?
The theatrical version was rated R by the MPAA. Later, an unrated version was released on DVD, adding almost 7 minutes of story and profanity to the film.
A detailed comparison between the theatrical version and the unrated version can be found here.