The last scene of Hamlet has a couple of these. Hamlet has one when Laertes informs him that he's been poisoned by the foil and he's got less than a half hour to live (a lot less, as it turns out). Claudius gets the next one when it becomes clear that the jig is up, Hamlet is finally sufficiently motivated to kill him, and no one's about to prevent it.
In The Taming of the Shrew, Lucentio has switched places with his servant Tranio for the purposes of a Zany Scheme. Eventually, Tranio is told to produce his father and gets a pedant to impersonate Lucentio's dad. Everything's going well until Lucentio's real father shows up, at which point Tranio, the pedant, and Lucentio's other servant Biondello frantically deny knowing him and try to get him thrown in jail. Just when it looks as if this might actually work, Lucentio arrives on the scene:
Biondello: O! we are spoiled and—yonder he is: deny him, forswear him, or else we are all undone.
Lucentio:[Kneeling] Pardon, sweet father.
Vincentio: Lives my sweet son?
[Exeunt BIONDELLO, TRANIO, and Pedant, as fast as may be]
In 1776, Franklin and then Jefferson realize that they're in serious trouble when Edward Rutledge asks for clarification about the anti-slavery clause in the Declaration. Franklin mutters "look out" when Rutledge first speaks, and Jefferson abruptly stands up when Rutledge starts to read it.