Historical clue formulations

• Heads-up in a review — 2024-05-31
• Important warning before you answer 32-, 40- and 52-Across? — 2022-01-13
• Warning you might give before revealing the endings of 20-, 29- and 45-Across? — 2021-03-10
• Warning for solvers of this puzzle — 2017-03-19
• Caution in a movie review, maybe — 2015-08-23