少し油断した隙に財布を盗まれた。
My wallet was stolen the moment I let my guard down.
格下の相手だからといって油断してはいけない。
Just because your opponent is weaker doesn't mean you can get careless.
油断大敵だ。最後まで気を抜くな。
Carelessness is your worst enemy. Don't relax until the very end.
ゴール直前で油断して、後続の選手に抜かれてしまった。
He eased off just before the finish line and was overtaken by the runner behind him.