In short, R contains silent coercions, vector recycling, lazy evaluation, partial matching on strings in various calls, exceptions to the rules, surprises, unexpected values, inconsistent and weird operations on edge cases and often chooses to continue on with these incorrect and bad values rather than throw errors and stop computation.

I'm not a fan of its writing style (there's a point where being funny hinders readability), but I believe chapter 8 of the R Inferno is replete with examples.

The combination of these factors results in a language where actually trying to reason about the veracity of a computation and coding defensively is exceptionally difficult, but people can load packages and make the wrong computations that look right exceptionally quickly, so they think of it as convenient.

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