Sometimes the biggest regret is not making a move, rather than making the wrong one.

Warren Buffett has mentioned his investment regrets multiple times. During the Clinton era healthcare reform policy impact on medical stocks, it was an excellent opportunity to buy at low prices. The same was true for Fannie Mae in the mid-1980s — a perfect value dip, but he missed it. These are mistakes that can't be seen on the books, each potentially costing billions of dollars.

But the most intuitive lesson comes from a young investment. He invested $2,000 in a gas station and ended up losing everything. It looks like a simple loss, nothing special. Where is the real pain? If he had chosen a different direction at that time, and compounded the growth until now, it could have grown to about $6 billion. This is the hidden opportunity cost — often more deadly than the direct loss.

In investing, the most expensive thing isn't what you did wrong, but the hesitation and absence when faced with opportunities.