So I've run into the exact same situation twice in the past couple months playing in blogger tournaments. Here's what happened:
I was in the small blind with A4 suited. It is pretty late in the tournament, last night there were eight people left and I was in 4th place with about $23k in chips, which was more than average. The blinds were 500/1000 with a 125 ante. I just call and bayne_s, who was in the big blind calls. Bayne was in 1st place at the time. There is $3k in the pot. The flop comes down A9A. I do the cute thing and check. Bayne bets 3000 & I just call. The turn was a 6. I check, Bayne bets 9000 (the pot) and I once again call. The river was a Q and I push in my last 9000 or so. Bayne calls and flips over A9 for a full house and I am out.
So, where did I goof this up?
Not raising preflop? It was a good but not great hand, so I figured I'd limp in & see what happens. I don't think Bayne was going to fold his A9 with any kind of "normal" raise.
Not betting out on the flop? I'm not sure betting out on the flop would have told me anything about his hand, but if he raised back into me, I would have known at that point he had something.
Not check-raising the flop? I think that this is what I should have done in retrospect. Raise it up to $9k from $3k and see what he does. If he calls I know I'm in trouble. The down side is if he's bluffing he folds and I don't get any more of his chips, which with 8 people left could be important.
Not thinking about what the turn bet meant? At the moment he made the bet, I wasn't thinking about why bayne would make a bet that large on such a scary board if he didn't have either 99 or Ax. I was in full donk mode pushing the chips in the middle.
Pushing on the river? I was somewhat resigned to pushing on the river no matter what happened. That was another dumb move, there was very little chance Bayne was going to fold if he had me beat and it was unlikely he was going to call my bet if I had him beat.
Usually I let these bad moves go not long after they happen. This one ate at me for a few hours after it happened, which did a number on how much sleep I got last night. I played pretty damn good tight (somewhat) aggressive poker for almost three hours and one bad play most likely cost me money, as I was only two people away from the money.