I don't disagree that it isn't necessarily smoking gun mistakes, but underperforming really is based on mistakes. Maybe too many penalties, or starting slow because 1st downs weren't converted or players being out of position, or dropping a pass early in the game that gets lost in the shuffle without us ever knowing if a drive could have been maintained and momentum could have been different without the drop.
In any case, the comment I was responding to was about an individual player (Romo), not a team, and does anyone really think Romo never made a smoking gun mistake or underperformed or faltered in a situation that should have been a forgone conclusion. Even the best QBs of all times have had few moments like that, so do we really believe Romo is the one exception?