Just for a moment, I lost my concentration and it cost me.

Party Poker No-Limit Hold'em, $ BB (9 handed)

Button ($14.75)
SB ($26.4)
BB ($24.2)
UTG ($31.8)
UTG+1 ($22.9)
MP1 ($24.75)
MP2 ($21.1)
Hero ($29.2)
CO ($20.3)

Preflop: Hero is MP3 with , . SB posts a blind of $0.1.
UTG calls $0.25, UTG+1 calls $0.25, MP1 calls $0.25, 1 fold, Hero calls $0.25, 3 folds, BB checks.

Flop: ($1.35) , , (5 players)
BB checks, UTG checks, UTG+1 checks, MP1 bets $1.35, Hero calls $1.35, BB calls $1.35, UTG folds, UTG+1 folds.

My decision to slowplay my set was a big mistake. If I had been paying attention, I could have folded this - that said, I did think I had the best hand.

Turn: ($5.40) (3 players)
BB checks, MP1 checks, Hero bets $4, BB calls $22.60 (All-In), MP1 folds, Hero calls $18.60.

Calling might have been the right decision w/o knowing what he had, but I did not give it enoght though first.

River: ($50.60) (2 players, 1 all-in)

Final Pot: $50.60

Results in white below:
BB has 5d 3d (straight, five high).
Hero has 2s 2h (three of a kind, twos).
Outcome: BB wins $50.60.