This is my biggest beef with R. It is constantly changing the dimensions and types of your data without telling you. Want to grab some subset of the rows of a matrix? Better add some extra post-processing in case there's only one row that satisfies your query, or else R will change its type!
The solution is not to make the programmer memorize obscure edge cases.
> x <- data.frame(foo=1:5, bar=1:5, baz=1:5)
> dim(x[,'foo'])
NULL
> dim(x[,c('foo','bar')])
[1] 5 2
> dim(x[,'foo',drop=FALSE])
[1] 5 1
> x <- dplyr::data_frame(foo=1:5, bar=1:5, baz=1:5)
> dim(x[,'foo'])
[1] 5 1