Below, inner_join() matches two rows that each have an NA for the key. This seems to violate the intuition about NA's (and leads to an incorrect result).
This seems to happen for all of the joins.
library(dplyr)
songs <- data_frame(song = c("Do-Re-Mi", "A Spoonful of Sugar"), movie = c("The Sound of Music", NA))
songs
## song movie
## <chr> <chr>
## 1 Do-Re-Mi The Sound of Music
## 2 A Spoonful of Sugar <NA>
singers <- data_frame(movie = c(NA, "The Sound of Music"), singer = c("Arnold Schwarzenegger", "Julie Andrews"))
singers
## movie singer
## <chr> <chr>
## 1 <NA> Arnold Schwarzenegger
## 2 The Sound of Music Julie Andrews
songs %>% inner_join(singers, by = "movie")
## song movie singer
## <chr> <chr> <chr>
## 1 Do-Re-Mi The Sound of Music Julie Andrews
## 2 A Spoonful of Sugar <NA> Arnold Schwarzenegger