I am not a lawyer or even particularly well-read on this specific scenario, but my read of the spirit of the law is that (if it indeed contains laxatives) it might still be illegal. What matters is not whether you sprang a surprise to one-up someone who was violating your rights or they had a fair chance to know what exactly the consequences would be, but that you deployed the laxative with the intent of harming the thief, and most modern legal systems do not like vigilante violence (even if it's "cute" vigilante violence like giving someone diarrhea). For an intuition pump, I'm not sure there is a significant difference between the note in question and "Warning! If you take this sandwich, and I know who you are, I will personally hunt you down, wrestle you to the ground and force-feed you laxatives!" as far as the law is concerned, but would you expect to get away with this act just because the target previously stole your sandwich and you warned them?

(On the other hand, I feel like the zappy USB port may actually be easier to get away with, especially if you say your threat model was corporate spies or criminals trying to steal your password, because "violence against tools" does not seem to be put in the vigilante violence box. Those special materials they have for safe doors that are designed to damage angle grinders (https://www.newscientist.com/article/2249275-material-that-c...) are not illegal.)

I know the old laxative in the fridge item has been a stand by in office etiquette enforcement for longer than I've been alive.

The court follows the principle of clean hands. A thief is not going to have a compelling case against someone when they are throwing the first stone by stealing someone's sandwich. It's an interesting take, and I'd have to dive into actual case law to even determine if the test scenario is apocrypha or not. However, I doubt the laxative in sandwich argument would be compelling to a judge when there are much more relevant and easy to reach challenges to overcome.

-I.e. State mandated weakness to exploitation by Law Enforcement (You can't defend yourself from exploits used by Law Enforcement, which you are also not allowed to know about under Executive Privilege) -The ability of the Government to ensure their tooling meets chain of custody preserving standards Etc...

As usual, not a lawyer, just read a book on legal research and reasoning once.

Yes. I expect that if I made myself a Miralax sandwich and labeled it with a huge caution label that I would be fine if someone decided to eat it anyway. It would be trivial to show that I had no intention of harming someone.

You would probably still be sued anyway, because why would a reasonable person make a laxative sandwich? The note is obviously a joke.