1. My wife and I first met each other by chance. She was a friend of a friend of mine.2. My wife's and my first meeting was quite accidental.3. We just found that our competitor's latest product's design is identical to ours. We are not sure if it is by accident or intentional.4. I usually drink beer. Occasionally I drink wine.
 by chance by accident accidental occasionalby chance, by accident, accidental 都是形容偶然，不期而遇，湊巧的意思，而 occasional 是偶爾的意思。