when you're trying to tell someone something and they just keep talking over you before you can finish

You finally get a word in after five minutes of silence, open your mouth, and they immediately start talking about their dentist appointment—the one they've already told you about twice. You wait for a natural pause that never comes. Eventually you just nod along, memorizing your important news to take to your grave. Later they'll ask why you've been so quiet lately.
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when you're trying to tell someone something and they just keep talking over you like you didn't even open your mouth

I tried to tell my boss about the website being down. Twenty-three minutes later, he'd explained his entire theory about why millennials don't use fax machines, and I was nodding so hard I think I gave myself whiplash. When he finally paused to breathe, I said "the servers are on fire," and he said "exactly, no work ethic." I've decided the website can stay down. It's earned a rest.

when you're trying to have a conversation and someone keeps talking over you like you weren't even speaking

You ever notice how the person talking over you always waits exactly zero seconds after you start a sentence before launching into theirs? It's like they've got a motion detector set to "vowel sound detected—INITIATE INTERRUPTION." I watched a guy do this to his wife for fifteen minutes straight, each time nodding enthusiastically like he was finally about to acknowledge her point, then immediately pivoting into a completely different tangent about his gym membership. At one point she just opened her mouth, he sensed the air movement, and cut in with "BUT ANYWAY." I'm pretty sure she's still waiting for that sentence to finish.