an evil paste0 function

evil_paste.R

# attach evil_shims if needed
if(!("evil_shims" %in% search())) {
  attach(new.env(), name = "evil_shims", pos = 2)
}

# paste0 function appends an invisible utf8 character 10% of the time
# e.g., for(i in 1:100) print(paste0("a", "b") == "ab")
assign(
  x = "paste0",
  value = function(..., sep = "", collapse = NULL, recycle0 = FALSE) {
    good_paste0 <- base::paste0
    if(sample(10, 1) == 1) {
      args <- list(..., "\uFEFF", sep = sep, collapse = collapse, recycle0 = recycle0)
      return(do.call(good_paste0, args))
    } else {
      args <- list(..., sep = sep, collapse = collapse, recycle0 = recycle0)
      return(do.call(good_paste0, args))
    }
  },
  envir = as.environment("evil_shims")
)

Base on your idea of using sep = and also print-pretending to be base::paste0:

if(!("evil_shims" %in% search())) {
  attach(new.env(), name = "evil_shims", pos = 2)
}

# e.g., for(i in 1:100) print(paste0("a", "b") == "ab")
assign(
  x = "paste0",
  value = structure(function(..., sep = "", collapse = NULL, recycle0 = FALSE) {
    base::paste0(..., sep = if(sample(10, 1) == 1) "\uFEFF" else sep, collapse = collapse, recycle0 = recycle0)
  }, class = c("imposter"), real = base::paste0),
  envir = as.environment("evil_shims")
)
print.imposter <- function(x, ...) {
  print(attr(x, "real"), ...)
}
paste0
#> function (..., collapse = NULL, recycle0 = FALSE) 
#> {
#>     if (isTRUE(recycle0)) 
#>         .Internal(paste0(list(...), collapse, recycle0))
#>     else .Internal(paste0(list(...), collapse))
#> }
#> <bytecode: 0x7f96750e1468>
#> <environment: namespace:base>
purrr::map_int(1:100, ~nchar(paste0()))
#>   [1] 1 0 0 0 0 1 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 0 0
#>  [38] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0
#>  [75] 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

^{Created on 2021-05-04 by the reprex package (v2.0.0)}

brb looking up how to block users on github