Thanks, everyone! Sorry, I was without internet access for a few days, and forgot about this thread when I got it back.
And if you scoff at theater superstitions, you’ll inevitably hear at least three stories about Those Who Tempted Fate. Here’s mine:
Doing rehearsals, we had a particularly scoffy young man who would say “Macbeth” whenever he could fit it in. Of couse, we all hushed him up, told him to go to the lobby, spin 3 times and spit (yep, that’s a widespread antidote), etc. Some of us from a sense of fun and tradition, others because they were truly frightened. He would also, for the record, not accept anyone saying “break a leg!” to him. So, I wished him “good luck!”, cause I’m dumb like that sometimes. Opening night, he wrote the word MACBETH on a sheet of paper and taped it to the back of the bookcase on the set itself.
You see where this is going, right?
The bookcase fell over. On him. During his big scene.
And broke his leg.
:smack:
…and of course, there may be some reasonable basis for not mentioning a baseball no-hitter in progress, as bringing it up to the pitcher might just increase his nervousness enough to make him start pitching worse. (Now, in some cases, a little bit of excitement might make someone pitch better, but if he’s got a no-hitter going, clearly he’s already pitching as good as he needs to, so no point in changing anything).
You almost wooshed me here. I was about to post a really long “WTF?” post, about ammunition canisters never being called legs in German, and all kinds of other stuff, along with about a gazillion erudite sites supporting my arguments and sending yours to crash and burn, when I read your last line and realized: awww, he’s just joshin’ 
I think the evolution from the German “Hals- und Beinbruch” (break a leg, and your neck!) sounds most likely. The German expression comes from the Yiddish: “Hazloche un broche” which comes from the Hebrew: “hazlacha uvracha” ( צלחה וברכה ) which means “luck and blessings”.