This is a mistake too. I always sell below ICO price if:
1) The whole market rotated to downtrend. Your newly bought tokens will lose too much of its price but who knows when it recovers if it will be at all. All that time your money will be locked (for what purpose?) instead of work again and again in new ICOs or trading. You will gain much more profit for this time than lose one time.
2) If first stage of ICO project is over but price is still below ICO one. A project has been listed on 2 exchanges, all hidden features been already released by the team, all partners were announced and now its time for long-term development of mainnet/product/dapp and etc... And the price is still below? It's time to sell, for sure.