Failure doesn't happen very often in D&D. It's not really something we enjoy, after all. Players feel like a combat encounter was close if one or two of the PCs was unconscious and dying for a round or so -- nevermind whether the actual outcome of the encounter was ever really in doubt.
Here's another statistic from my trademarked "Bureau of Totally Believable But Made Up Statistics": less than 1% of all combat encounters of D&D that are played result in a true "failure", a TPK (Total Party Kill).