zsh does not split an unquoted $VAR into words, so a variable holding several arguments passes them as one

Fact

With the SH_WORD_SPLIT option off, as it is by default, zsh expands an unquoted $VAR to a single word. After SIZES="800x600 1024x768", the command prog $SIZES gets one argument, 800x600 1024x768; bash, and zsh in sh emulation, split it into two.

Why it matters

Nothing fails in the shell. The program receives one argument where it expected a list, and the error surfaces inside it, far from the cause: a parse that yields NaN, a lookup of a file named with spaces, a loop that runs once. One-liners and scripts written for bash break this way under zsh, the default shell on macOS.

How to apply

• Keep lists in arrays, which expand the same in bash and zsh: sizes=(800x600 1024x768); prog "${sizes[@]}".
• In zsh only, ${=VAR} splits one expansion, or setopt sh_word_split splits them all; or run the script with bash explicitly.
• See also [[zsh-equals-word-is-a-command-lookup]] and [[unquoted-url-fails-in-zsh]].