It’s okay to hope your dice roll well. It’s okay to hope your opponent’s dice don’t. (You have my permission.) What’s never okay is to shout out that sentiment while your opponent is making a roll.
Shouting “FAIL!” “ONES!” “DIE!” “MISFIRE!” (all things I’ve actually had shouted at me) or anything along those lines isn’t actually going to make that result any more likely. The only thing it’s going to accomplish is make you look like a complete asshole.
This is a jerk shibboleth to me. If someone does it once, I give them the stinkeye. People get caught up sometimes, though; I judge… but not a lot. If my opponent does this once, I’m having a bad game.
Twice, though? Twice is repeated behavior. Twice is a pattern of douchebaggery. I generally call opponents on it if they do it a second time. “Really? Really?!?! Did you actually just cheer for me to fail that roll?” If my opponent does this more than once, I’m having a terrible game and have absolutely lost my cool.
To their credit: I’ve never called someone on this and not have them apologize for being a jerk.
While I wouldn’t begin to suggest that I see this happen regularly; it’s happened to me a handful of times, and I often hear about it happening to other people.
(I try to cheer for my opponents to roll well… I’d rather win a game because I won it, not because my opponent blew some crucial roll. And, if they do blow it, I’m quietly thankful, not celebratory. Of course, I also tend to feel pretty bad about total bullshit rolls on my part. Stupid shame-based religious upbringing.)