1.If IPO is alloted,he tries to cash on first day and thus misses big move.
2.If buys a share and as per Murphy's law that share has to go down and moment it recoups and it reaches its buying level,he sells feeling relieved that his loss has been recouped.Now is the time big move starts.So donot panic if share falls below your acquisition price.
3.The above philosophy is also true for shares you sell/short sell.
4.To have big gains sit tight on fundamental companies and let the breakout occur.