Well, I was trying to duplicate it by adding a few more characters, but then the B's stopped appearing, and they still weren't there when I tried again. In any case, here's what I have for the text string now, which I think is identical to what it was then but doesn't seem to be working anymore:
(The effect was indeed reproduced multiple times after first showing up, but it got lost when I tried to edit the level. Should have made a backup...)
Let's see, random bits of information... prior to the incident, I had gone through three different texts, none of them set to Vanish, and none containing character problems. I had one heart, and at one point a bubble shield but not anymore. That's probably all that might have any impact... my only notion is that it might have something to do with the numbers employed by the shield, but I don't really know.