Consider the following situation:
$84 SNG on Full Tilt Poker. Blinds are t30/t60.
Hero (BTN) has t1000. Villain (SB) has t2000
Pre Flop: (t90) Hero is BTN/SB with Td 2d
Hero calls t30, BB checks.
Flop: (t120) 9d 8c Jc
BB checks. Hero checks.
Turn (t120) 2s
BB bets t120. Hero calls t120.
River: (t360) 6s
BB bets t180, Hero calls t180.
Final Pot: t720. Hero mucks Td 2d. BB shows 3d 6h. BB wins t720.
Preflop seems okay, but minraising is okay here too. However, when the flop comes out, we have to bet. Even if to protect our equity when he has a worse hand, he'll bet the turn very often and unless you plan on jamming over a turn bet (which is an overbet), just bet the flop. If our opponent is capable of check-raising then folding to a jam with a hand like Jx here, checking the flop is a very bad play.
On the turn, our opponent bets with some combination of two cards. He's made a pot-sized bet, but this doesn't necessarily narrow his hand down. He could have no pair, top pair, middle pair, a straight draw, a flush draw, or ace high. All of these hands would be betting to protect their equity from the river card. He could also have a bluff. In this case, he's betting to get you to fold. Calling is fine with our bottom pair and straight draw.
On the river, the opponent has bet half pot. He's indicating that he has a hand, but since this is the first time he's indicated that, we only have one decision to make: call or fold. We call if we think our bottom pair wins more than 25% of the time. There is no right or wrong decision to make against an unknown villain, only your reads can tell you whether trying to catch a bluff here is the right play. In this case, we put the opponent on a bluff and realize that he was actually making a mistake betting the river (a bet which will get called by stronger hands more often than by weaker hands), but there's nothing we can do about the fact that he picked up a six and has us beat.