I guess I have to write in this thread too, since I certainly have some relevant experience. I haven’t been investing for long, but I’m already looking back at my first months’ purchases in horror.
An easy single example is Harvia, which I bought for 500 euros in the summer. The price kept falling, and being the “wise” one, I thought “it can’t drop anymore, it’s already fallen so much.” Well, I bought 500 euros more, and the price dropped about 30% over the next two months. At that point, I sold everything and admired the handsome 300 euro realized loss in my equity savings account (OST). Everything completely fell apart here. My background research was barely looking at target prices and checking the price curve on Google Finance. It’s also highly recommended to put all your eggs in one basket, especially without any knowledge of anything! Fortunately, I’ve learned at least something from this, and those losses have already been covered 
. Harvia has also returned to my portfolio, and luckily with slightly more sensible multiples/price.
What else has there been… hmm.. well, Embracer, of course. Good grief, what did I do again. FOMO hit my soul directly and I joined the ranks of many buyers around 49 crowns. I topped up again at 47 crowns. Well, on the same day, I think it hit 42 crowns, if I remember correctly. I didn’t sell at a loss though; I actually managed to get rid of the shares with a slight profit. A good trade, right? Well, it could have been, if the reason for buying had been anything other than running after everyone else headlong. I’ve thought about this stupidity many times when I’ve felt like jumping on, for example, the Canada train (I’m sure you know which ticker I’m talking about). I wish you success with that company, of course, but it doesn’t belong in my investment plan, so I’m staying away (at least for now 
).
Those were a few individual examples, but what about in general? Well, I have experiences/thoughts on that too. I’ve been watching the price action way too much, which has led to “useless” flips. I’ve seen how some stock has dropped a lot, and then thought “now is the time to strike this company whose valuation I don’t understand, whose numbers I don’t understand, and I don’t even understand the company’s operations or even the industry.” Then I’ve bought and held for maybe a week and sold it forward, only enriching the broker. Luckily, since December, I’ve managed to get some sense into this business, and I’ve only been adding to positions I already own or opening a couple of new lines. Good for me!
This last one is a bit tongue-in-cheek. Thursday, Jan 12, 2023, I was talking to my American friend about Tesla. The price was around 125 dollars. We had a pretty heated discussion about Tesla’s situation, and the result was that I told him I’d buy Tesla the next day. Well, on Friday the 13th, Fortum, which I had been following for a long time, “dipped” (maybe 50 cents), and I opened a position for it in my portfolio. My friend then bought Tesla, but I was confident in my decision. It’s not like Tesla has even gone up 100%, so it would have been totally pointless to put my money there. 
I know it’s always easy to be wise in hindsight, and I can’t do anything but laugh at this situation afterwards. But I could have trusted myself and my own analysis even a little bit. There have been mistakes and surely there will be more in the future, as I’m still at the beginning of my investing career. It’s been fun, though, and it really puts the brain to work. 