What it means
た拍子に describes an unexpected event that occurs at the exact moment another action or change takes place. It often highlights an accidental or unintended consequence, such as something falling, breaking, or being triggered. It attaches to the plain past form of a verb and is more precise than a general expression for simultaneous events.
Usage note
た拍子に normally describes an instantaneous, often accidental result rather than a planned sequence. In a polite sentence, the preceding verb remains in the plain past form before 拍子に, while the main clause can take a polite ending.
How to use it
強風で窓が開いた拍子に、棚の上の花瓶が床に落ちました。
When the window opened in the strong wind, the vase on the shelf fell to the floor.
強風で窓が開いた拍子に、棚の上の花瓶が床に落ちた。
When the window opened in the strong wind, the vase on the shelf fell to the floor.