So if you know the punch line to a joke someone is telling you, what’s the proper etiquette? I’m mostly thinking of jokes that start off with a question – like, “What do you call a steer that ate a bomb?”
Any of the above, depending on the situation (and whether the joke is still funny).
If I’m not the only person in the audience, I definitely don’t blurt out the punchline - is about the only rule, IMO.
Other – it depends on the teller and the crowd, but as a rule I’ll say something like “Ah, yes, I know that one :)” - and then let the teller butcher it themselves 
So… what do you call a steer that ate a bomb???
Abominable!
Specifics will depend on the case, but along the lines of “laugh if you find it funny, whether you’ve heard it before or not”.
It wouldn’t be the first time the punch line turns out to be a different one than the one you knew, also.
Like the others said, depends on the joke and the situation.
Another vote for “depends on circumstances”, but I tend toward “let teller finish joke, muster some expression of amusement even if not entirely sincere”. Seems the kind thing to do.
From an etiquette view you let people tell their joke. Exceptions abound but that is the general rule.
More tricky are the twists on jokes that have a question (What’s the difference between…) that are turned on the listener (“Do you know what that sound is when a woman orgasms?” “No what?” “I didn’t think you would.”). Contrast that with when the joke needs a serious attempt to give the punch line (“What is a pirates favorite letter?” “Aaarrrr” “No, 'tis the sea”)
Depends on the teller. Sometimes I go along, sometimes it’s more, “Oh right, your mom told me that one” (when a joke is particularly offensive).
Usually something like this.
“What do you call a steer that ate a bomb?”
I look excitedly around and say to the others there, “Oh this is great, you’ll love it. Best joke I’ve heard for ages,” then back to the joker, “So what do you call a steer that ate a bomb?”
“Abominable.”
“Oh. Sorry. My mistake. I haven’t heard that one before after all.”
After a beat or two, “Are you sure that’s the punchline?”
My steer has no nose!
How does he smell?
Terrible!
So many people SUCK at telling jokes.
If somebody starts to tell me a joke I’ve heard and I know that they’re going to destroy it, I just say something like “Oh, yeah that’s a good one!”
They usually don’t get the hint and keep telling it, though.
It’s always a dick move to ruin the punchline if you’re not the only one on the receiving end. What frustrates me even more is when somebody will take a stupid guess and throw out a wrong punchline. Totally takes the wind out of my sails.
Did you hear the one about the one-armed basketball player?
He has no left.
Did you hear the one about the one-legged basketball player?
His jumper is a foot short.
It best to just let someone have their moment to be funny.
I’ve used “that’s an old one but a good one” lines and felt like a passive aggressive jerk for it.
Might use it if someone is being bothersome with joke telling.
However, if I can come up with an answer that will derail a joke, I’m going for it. It’s fair game.
Yep, depends on the joke teller. If I know them very well, I’ll say the punchline and something like, “that was funny in 1995.” If I do not know them well, I’ll let them finish and give a chuckle.
You…are cruel. And I am so stealing that.
This is an important point. Unless you’re really good at it, joking should remain between friends, and conventional etiquette rules don’t really apply. If a stranger tells me a joke I’ll listen and chuckle at the expected punchline, or give them a big laugh if they have something really funny that I haven’t heard before. But most of the classic joke telling I hear comes from people I know fairly well, and it’s kind of a merciless competition. You better be original and tell it well to expect to get to the end and get a laugh. Also, it’s a lot easier when the other guy is drunk. Get you timing right and anything can sound funny.
I picked Other - it depends on who is telling the joke and whether I’m the sole audience or if I’m in a group. I’ll stop my husband or my sibs and certain friends if I know the joke and I’m alone. I’ll suffer thru my FIL’s telling, since he gets his feelings hurt easily. If I’m in a group, I’ll offer a chuckle if it’s funny, or a smile if not.
If it’s someone I know well, and they are only talking to me (and not a group of people including me), I’ll stop them and tell them I know the joke already. I will probably do this even for someone I don’t know all that well.
In any other circumstance, I’ll let them tell their joke and laugh in remembrance.