data.frame(data.frame(numbers=c(1,2,3)), data.frame(letters=c("a", "b", "c"))) makes a data frame:
numbers letters 1 1 a 2 2 b 3 3 c
data.frame(data.frame(numbers=c(1,2,3)), data.frame(letters=c("a", "b", "c", "d"))) does not:
Error in data.frame(data.frame(numbers = c(1, 2, 3)), data.frame(letters = c("a", :