1) “Jane, nice to see you. So you say you can only stay a few minutes?”
Before Jane can open her mouth, the person she is with says “She has a long drive and she’s nervous about getting caught in rush hour traffic”. Then when the other person takes a breath Jane says “I’m not nervous. It’s just that I have to stop a couple more places before getting on the road.”
2) “OP, you said in your first post that your mother has trouble getting the items from the garage. Any particular reason?”
Someone replies “I think what the OP probably means is that her mother is elderly and blah blah blah yada yada yada.”
Later OP comes back and answers the question and it turns out age is not a factor.
3) “Bob, do you like apple pie?”
Someone jumps right in with “He likes it but he’s trying to lose weight.”
How do you handle this?
When it comes to #2--something that might happen on a forum like this one--I think it's not so bad. It's not "real time" and the question might just hang there if the OP doesn't come back for a while, and then the OP might miss it anyway if there's a lot of new replies. Of course, everyone who speculates risks looking silly if the OP comes back and says, "Actually, that's not the issue."
I think the best people can do is try to give measured replies that don't assume facts not in evidence, so to speak--maybe say, "I don't want to speak for the OP, but I was imagining that it was because her mother was elderly, in which case I would suggest... But, that may not be relevant here." I think that forum threads aren't just for the OP, they're for everyone reading them (whether posting or not), and someone's speculation may be helpful to a lurker who has the same problem, except her mother really is
elderly. That shouldn't be an excuse to stray wildly off-topic or put words in someone's mouth, of course.
I think in the case of #1, there's no need for "you" to really do anything, as Jane starts speaking for herself when she gets the chance, and corrects the other person's information. If Jane and the other person want to have words about it later, that's their business and not anyone else's.
In the case of #3, I would also assume Bob is capable of speaking up for himself if he wanted to, whether to correct the other person or agree with them. If the question was more like, "Bob, would
you like some apple pie?" that is, the answer affects my actions (to serve him a piece or not), I would probably kind of nod in acknowledgment of the third party, but then look back at Bob questioningly for the final response. That is, I wouldn't put the pie away without hearing from Bob directly on the subject.
For #1 and #3, I will admit that such behavior would probably give me a somewhat negative impression of the third party, especially if they did it a lot. But, I wouldn't see it as my place to jump in and defend Jane/Bob or correct the situation somehow. At most, if the situation warranted it, I would (as in #3) continue looking expectantly at the person I had originally addressed. That's just me, though.