Franny's Mother, played by Joanna Gleason, sings very badly in the movie and is asked numerous times by her daughter not to sing. In fact, Gleason has a very good singing voice and has appeared in several Broadway musicals, including "Into the Woods" for which she won the 1988 Tony Award for Best Actress in a Musical.
In an early scene in Matthew McConaughey's office, he indicates a framed picture of his "sister". The prop is actually a picture of associate producer and choreographer Anne Fletcher, who had a scene in the movie that was cut.
At the first wedding, Jennifer Lopez's character Maria comforts the bride by complimenting her great thighs. This may be a small reference to the huge media focus on Jen's "big" thighs when she first began her singing career.
The trivia item below may give away important plot points.
In the beginning when Mary is giving Penny reasons why she doesn't want to be with Massimo, she says he kept asking her if she had a vagina. Penny says maybe he wanted to be a doctor. Justin Chambers who plays Massimo later plays a doctor, Dr Alex Karev, in Greys Anatomy