Harry tested that. He concluded that the tea works the other way. That is, it detects when something weird is going to happen and gives you an impulse to drink.
Harry tested whether you could make something weird happen by drinking the tea, not whether you could make someone drink the tea when you did something weird, subject to you not yourself knowing what would be weird enough before you saw them drinking.
I have refined my idea based on some re-reading. I forgot that you aren't guaranteed to choke on every sip. Someone let me know if the following is too farfetched.
As you said, when something shocking is going to happen, someone with access to the tea feels an impulse to drink it. Now recall this line from chap...
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.