The R Inferno

I recently came across The R Inferno written by Patrick Burns. In a satirical style following Dante's Inferno, he discusses pretty much every stack exchange question I've ever looked up.

The R Inferno

 Among the many new things I learned and immediately implemented: 

• the amazing "drop = FALSE" argument that prevents subscripted matrices from dropping a dimension and becoming vectors

As Burns so helpfully repeats twice on page 67:

"NOTE: Failing to use drop=FALSE inside functions is a major source of bugs. "

Suppose you have a matrix my.Matrix and you want to take some subset my.subset of the rows in M.
    > M[ my.subset, ]
If my.subset has length 1, then the above code will return a single vector.
    > M[ my.subset,  , drop = FALSE]
Even if my.subset has length 1, the above will return a matrix with a single row.