Don't Let Someone Else's Frustration Lead To Your Frustration Too
Frustration spreads. When somebody is worked up, they often go hunting for a target to pin the blame on, or they just want company in their misery. The catch is that they usually want you to agree with them, and if you don't, you become the next thing they're angry about. So now there are two frustrated people, and the original problem is still sitting there untouched.
For some people, handing the blame to someone else is how they close the matter. They decide the other person caused it, whether that's true or not, and then they can walk off. Sometimes they leave muttering, a relationship wrecked on the way out. And if the other person in the exchange plays along, they end up feeling guilty too. Every so often, once there's a little distance between the person and the moment, they'll realize the other party never actually did anything except try to sympathize.
I've put it this way before: don't let legitimate frustration breed more frustration than existed to begin with. Offering help is a good instinct. It stops being useful when the person has no real intention of weighing what you say. Plenty of people in that state aren't looking to be talked down or to decide, in hindsight, that the thing wasn't such a big deal. You may already have trained yourself not to let one frustration pull another one behind it. They haven't learned that yet.
If someone looks like they've got a storm cloud parked over their head, don't go inviting the lightning, because it can swing around toward you. I've learned to keep my distance. Lightning lands closer than you'd guess, so the moment you spot even the chance of a strike, move along. The classic trap is the checkout line. The person ahead of you has a problem, and it makes no difference how many people stand between you and them. Say something kind or something sharp, either one might land wrong. Sometimes the only cure is letting the storm pass on its own. If you're near the back you might slip away, and the smarter move is to never get in that line once you see trouble building, or already underway.
It gets harder when the trouble is already happening and you can't stay out of it. Your husband or wife climbs into the car with you, or you have to get in to drive home, and they're already furious. There's nobody else around to aim it at, so it lands on you. Whatever you say seems to make it worse. You'd rather duck or bolt.
Probably the better approach is to wait until everyone has settled and bring it up then. Something like: remember the day I got in the car steaming about how a salesperson treated me, and I dumped the whole thing on you. Could we agree, for either of us, that when this happens we'll both cool down and talk it over later, somewhere safe to have that kind of conversation? You might even point the other person toward some of these notes on the problem.