I had thought to use homographs. Sadly, all the models I tried were able to decode something like:

"ﾌㄖ乇ㄚ ᗪㄖ乇丂几'ㄒ 丂卄卂尺乇 千ㄖㄖᗪ"

However, I have noticed that voice assistants have a hard time understanding homonyms. Saying "bow" (as in to bow one's head) is often stored as "bow" (as in a bow and arrow). I wonder if there's a sufficiently complex sentence which is intelligible to humans but not to machines?

There's garden path sentences, where the sentence is phrased in such a way as to cause you to misparse the sentence when you first read (e.g. "The old man the boat"); but those typically confuse humans (I'm not sure how effective they are on LLMs).

Relevant xkcd: https://xkcd.com/2793/