Panic Mistakes:
1. Sold 15k worth of stock near its lowest point in at least 10 years. Within 1.5 years it would have been worth at least 45k.
2. Also turned off investments in that stock while it sat at a low point for a couple years. Would have gotten more stock currently worth another 50k or so during that time period.
3a. Decided to invest in individual stocks with my rolled over 401k.
3b. Decided to sell said individual stocks when they dropped heavily over the next couple weeks. They went right back up a week or so later, but I'd lost around 10% on my money. It took awhile to gain that back.
Just not thinking mistakes:
1. This is a good one. Decided to buy and put money down on a new house on a quick 1 day decision. It was "Hey nice house, quick lets put money down before it's gone" (new construction). Here's the thinking that lead to and resulted from this decision:
a. We don't want to put kids in day care, so if we're going to have kids, one of us needs to quit their job.
b. We might not want kids, so we need to decide before moving.
c. Ok, this house we're looking at seems great for kids, it's huge, has 4 bedrooms, 2.5 bath, full basement, etc. Lets buy it quick before the price goes up anymore.
d. We can't have kids because we can't afford the house on only one salary. We can't move because it's new construction and pre-owned home prices aren't going up in the area (and are selling slow). We'd kill our equity with Realtor commissions. It's just shocking looking back that we could have been so dumb
2. Invested wifes entire 401k in her company stock when her company stock fell on a spur of the moment decision. Was saved from stupid mistake by her company stock rising the next day and I was able to sell for a nice profit. Learned never to do that again while actually gaining money
That's about all I can think of right now.