Switcheroo / Fallout

What ensues when a character makes an error?

What ensues when a character makes a minor error, such as transferring to the wrong train or getting off an elevator on the wrong floor?

Or perhaps you want to make it a larger error, say sending an important (or simply private?) message to the wrong recipient. Or a terrible mistake with profound fallout, such as bombing the wrong house, operating on the wrong side of the body, drinking the poisoned tea instead of serving it to the intended victim.

Visualize the situation. What's the most dramatic moment here? Will you start with the intended action, the switcheroo itself, the moment the character realizes their error, or perhaps the aftermath?

Good writing.