As a poker player myself, I have witnessed many river "miracles".

The problem with statistics is there is ALWAYS a chance that someone will get a card to beat you. If you have the lead at the flop, you must BET like you have the lead. Failing to do so allows for someone to take odds that are favorable to beat you.

For instance, if someone has four cards to a flush on the flop (two clubs in the hole, two on the board), there is ALMOST a 1 in 4 chance they will make their flush on every card. Therefore if you think you are AHEAD and you have multiple opponents, you must bet MORE than 1/2 of the total pot to DENY them the correct odds (if you bet 1/2 they will have 1 to 3 odds, plus the potential to make more money off of you, easy to call). If you are NOT betting correctly, people will stay in the hand, they will beat you, and you will complain.

For a more in depth discussion of proper betting in no limit hold em, see Harrrington on Hold Em, a great book for both the tournament and cash game player ( although geared more for tournament hold em).