Missing a tactic and getting away with it
I have many weak points (I am a 1300-1400 player for the time being), and missing a tactic in the opening should have cost me this match in the Slow Swiss #4, 1st round:
A game where I should have been way behind early on. When that did not happen, this match turned into an interesting contest where the extra centre pawn was a decisive factor.
Playing in an OTB at the weekend, I was on the losing end of a similar kind of match; my opponent was a pawn up early on and slowly turned it into a clear win. My win was not nearly as easy, thanks to some good play by my opponent, who among other things was a lot better with his rooks early on. I blush when I see how long I played before moving any of mine!