| Clue | Source | Date |
|---|---|---|
| Unique events | Wall Street Journal | 22 Feb 2025 |
| Unrepeated instances | New York Times | 29 Oct 2023 |
| Events with a single occurrence | Universal | 22 Oct 2023 |
| Unrepeated events | Newsday | 28 May 2023 |
| Singular events | LA Times Daily | 22 Sep 2022 |
| You won't see them again | New York Times | 04 Apr 2020 |
| Custom-created products | ||
| Single occurrences, at Oxford | ||
| Single occurrences, at Oxford |