Recently, I reviewed a paper that introduces a package, so I reviewed the code and noticed that the authors often used && (and) and || (or) whereas only & and | were required. I guess this is due to:

1. other programming language;
2. the fact that for vectors of one element in R & and && are equivalent (same for | and ||).

Regarding point 1, we could take C as an example. In C, & and | are bitewise operators while && are || logical ones for instance 4 & 7 gives 1 but 4 && 7 gives TRUE. In R, things are a bit different: && and || only test the first element of two vectors:

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R> c(0, 0) && c(1, 0)
[1] FALSE
R> c(0, 1) && c(1, 0)
[1] FALSE
R> c(1, 0) && c(1, 0)
[1] TRUE

whereas & and | perform logical tests element-wise:

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R> c(0, 0) & c(1, 0)
[1] FALSE FALSE
R> c(0, 1) & c(1, 0)
[1] FALSE FALSE
R> c(1, 0) & c(1, 0)
[1]  TRUE FALSE

and a warning signal is returned when vector size do not match:

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R> c(1, 0) & c(1, 0, 1)
[1]  TRUE FALSE  TRUE
Warning message:
In c(1, 0) & c(1, 0, 1) :
  longer object length is not a multiple of shorter object length

Using && and || for vectors of 1 element may not be a big deal after all, but if you are actually trying to do element-wise logical tests and you use && and || it could easily generate errors that go under the radar, so you should better be aware of this!