All investors and financial experts get things wrong from time to time and learn valuable lessons in the process. In this Fool Live video clip, recorded on Nov. 3, The Motley Fool's own Certified Financial Planners®, Matt Frankel and Robert Brokamp, answer a viewer's question about things they used to think but now know better.

Robert Brokamp: Calvin asks, "What did you use to think that you now know to be wrong?" That's a really interesting question. I guess I would say that I figured it when I was younger than I would want to retire earlier. Now, as I am 52, and within about 10 years of the age and I thought I'd want to retire. I actually now think I could work well into my late '60s, if not '70s, as long as the Fool learning will have me around, maybe just part-time. It might be just part season. But I have done more. First of all, I know I'm lucky. I like my job, so it's easy for me to say that.

But also for my early retirement service and our answers podcast, I have just done a lot of research over the last year or two about how retirement actually may not be healthy for people. It's good to have reason to get out of bed. It's good to have intellectual stimulation. It's good to have social interaction, which is what we used to have. We all met in the office and I hope one day the Motley Fool office will be open again. I think that rather than retiring early like I thought I would do, I'm probably going to work at least part-time well into my '70s. All right, Matt, what do you got?

Matt Frankel: I would actually add to that that I don't necessarily want to retire early. I want to be able to retire early. It's such a good question that I want to add one thing that I've used to think was true. I used to think that there was nothing wrong with taking profits on a stock no matter what. Hitting the sell button if you were up and you couldn't lose. Then I sold Tesla (NASDAQ:TSLA) stock to pay for my wedding in 2013.

Robert Brokamp: Oh, boy. 

Matt Frankel: My split adjusted cost base is around $4 a share in Tesla and I sold it at a split adjusted $9 a share. Today, it's at about $1,200 a share. I told myself, it's fine to sell because there's nothing wrong with taking profits. I can't regret making an investment that doubled. That was dead wrong. But I learned from that and if I hadn't learned from that, I wouldn't have kept the Square (NYSE:SQ) stock I bought right after its IPO as long as I did. My wife wouldn't have kept her Etsy (NASDAQ:ETSY) stock that she bought for $6 a share till now. It was a hard lesson to learn and I literally could've bought probably five Model S's with the money I left on the table. But that's one thing that I used to think that I was definitely wrong about. There are good reasons to sell stocks. The simple reason of because they went up, is not one of them.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.