> I can't see why the error you describe would occur in this instance. Would you mind explaining?

Sure, it's due to pathname expansion. The most commonly used pattern is `*` (e.g. `echo *.txt` might expand to `echo foo.txt bar.txt`), but there is also `[...]` for matching a character range. If the current directory contains files named "i" and "j", then `tr -s [:alpha:] Y` will expand to `tr -s i j Y`, which is not what you want. (The reason it works when the current directory doesn't contain single-letter filenames is that a pattern which doesn't match any filenames is left as-is.)