I do this all the time. Both racing thoughts and flight of ideas and derailment can do that to you, where you're talking about one thing and it reminds you of something else. Your brain is jumping so fast you end up in a place completely different than what is being said to you.

I have a lot, and I mean a lot, of blocking in my speech. That's where you are talking and forget and just stop. So, first I have pressured speach, then I have thought blocking. So I talk too fast and too much and then suddenly dead stop with a blank stare. But, I can be reminded what I was thinking. I forget words all the time. Or confuse them with different words.