I once mentioned on the Usenet newsgroup
alt.aquaria in 1989 that I had a Plecostomus catfish die in my aquarium. Somebody else remarked theirs was fine. Shortly after, theirs died. Somebody else said their pleco was fine they had no problem keeping them alive. And then it died. After this happened a few times I pointed out that every time people bragged about their Plecostomus catfish on Usenet, then that fish would die a short time later. Similar to the way Macbeth is called "The Scottish Play" to avoid the bad luck of actually saying the name.
People began writing "pl*co instead of pleco to avoid the dreaded usenet-pleco curse.
That much I remember. I don't actually remember who spelled it "pl*co" first; it might have been me but probably not.
Postscript Dec 2014 - Nope, it wasn't me it appears it was David Wetchler in this thread. Dean Hougen documented this in 1996 as well.