Thanks for the answer, Suki.
Actually I don't think I explained my solution properly.
In my post, I used the correct [youtube][/youtube] tags around the video link. But I ALSO used just the word [youtube] in the body of the post.
When I managed to view the post (before it redirected to youtube), it showed a blank box where the video should have been. This blank box started where the [youtube] was used.
So it looked something like this:
Announcement concerning [youtube] tags. You can now use [youtube] tags to embed your youtube videos.
The end result was something like this:
[BIG BLANK BOX]
When I changed the [youtube] items in the first line of the first example, the big blank box disappeared, and the thread stopped redirecting to youtube, and the video showed as expected.
I hope that's clearer now. The problem doesn't exist any longer because I took the [youtube] away in the text of my post, and only used the tags around the video link.
So, the only bug that exists now (as far as I can tell and I'm no expert) is if someone uses the exact phrase [youtube] without a link, and without closing the tag with [/youtube], the entire thread will redirect to youtube.