A proverb meaning that a single example reveals the general pattern; one instance indicates what usually happens in other cases.
彼は時間にルーズだ。今回遅刻したのもそうだし、一事が万事だ。
He's sloppy about time. He was late this time too — one instance says it all.
店員の態度が悪かった。一事が万事で、その店では他の商品でも同じ対応を受けるのだろう。
The clerk's attitude was bad. From that one incident you can guess it's the norm and you'll get the same treatment elsewhere in the store.
小さな間違いを放置すると、他も同じようにずさんになりがちだ。一事が万事という考え方だ。
If you leave small mistakes unchecked, other things tend to become sloppy too — that's the idea of 'one instance reflects everything.'