I came across this by running revdepcheck::cloud_report() on shiny and it seems @hadley was seeing the same thing in r-lib/revdepcheck#288 and attempted to fix in #133, but that was never merged. In my case, I was seeing:

Processing package results  42% (ipc)
Error in readChar(path, nchars = file.info(path)$size, ...) : 
  invalid UTF-8 input in readChar()

It turns out the error was coming from this call to rcmdcheck:::get_test_fail() which in turns calls rcmdcheck:::read_char()

I can reproduce the error locally by trying to read in {ipc}'s testthat.Rout.fail file the same way as rcmdcheck:::read_char():

path <- "ipc-testthat.Rout.fail"
readChar(path, nchars = file.info(path)$size)
#> Error in readChar(path, nchars = file.info(path)$size) : 
#>  invalid UTF-8 input in readChar()

With useBytes = TRUE, I can successfully read the file, but another downstream failure happens in rcmdcheck:::get_test_fail()'s call to nchar() (I'm guessing this is why @hadley said #133 (comment))

txt <- readChar(path, nchars = file.info(path)$size, useBytes = TRUE)
nchar(txt)
#> Error in nchar(txt) : invalid multibyte string, element 1

However, if I change the encoding to UTF-8, then it works:

Encoding(tx)
#> "unknown"
nchar(enc2utf8(txt))
#> [1] 12252