This is one of the biggest leaks that I've helped my students to
find, and as these hands illustrate, it's not something I'm immune to
myself. I'm talking about choosing the wrong hands to defend against a
possible bluff/semi-bluff when there are still cards to come. It is so
much more important to defend with draws that have good equity against
your opponent's range than with hands that are "made" (which is really
a meaningless concept anyway).

Full Tilt No-Limit Hold'em, $20.00 BB (2 handed) - Full-Tilt Converter Tool from PokerSavvyPlus.com

SB ($2060)
Hero (BB) ($2239)

Preflop: Hero is BB with 9, 9
SB bets $60, Hero raises to $200, SB calls $140

Flop: ($400) J, 3, J (2 players)
Hero bets $244, SB raises to $520, Hero raises to $2039 (All-In), SB calls $1340 (All-In)

Turn: ($4120) 10 (2 players, 2 all-in)

River: ($4120) A (2 players, 2 all-in)

Total pot: $4120 | Rake: $0.50

Results:
SB had Q, J (three of a kind, Jacks).
Hero had 9, 9 (two pair, Jacks and nines).
Outcome: SB won $4119.50

Full Tilt No-Limit Hold'em, $10.00 BB (2 handed) - Full-Tilt Converter Tool from PokerSavvyPlus.com

SB ($3011)
Hero (BB) ($2010)

Preflop: Hero is BB with J, K
SB bets $30, Hero raises to $111, SB calls $81

Flop: ($222) 4, 4, 8 (2 players)
Hero bets $144, SB calls $144

Turn: ($510) 3 (2 players)
Hero bets $355, SB raises to $850, Hero calls $495

River: ($2210) 4 (2 players)
Hero checks, SB checks

Total pot: $2210 | Rake: $0.50

Results:
SB had J, A (three of a kind, fours).
Hero had J, K (three of a kind, fours).
Outcome: SB won $2209.50

Just look at hand 2 for a second. The results throw things off a
bit, but let's suppose Villain is planning on folding to a turn shove.
If I shove with 99, he folds worse hands like AJ and it doesn't matter
what I had. When he does call, I am drawing to 2 outs. If I shove KJs,
I apparently bluff him off some better hands and also I will usually
have 8+ outs (though occasionally I'll be drawing dead). Even without a
flush draw, two big cards like AK is a better hand to shove than 99.