た弾みに / た拍子に【たはずみに / たひょうしに】

た拍子に and た弾みに say that the moment one thing happened, it set off something nobody intended. The verb before them is in the た-form and is usually a sudden movement – standing up, turning round, sneezing, tripping – and the second half is the accident.

Because the result is an accident, the second half can't be something the subject chose to do. It often ends in てしまった.

• × 立ち上がった拍子に、窓を開けた → 立ち上がって、窓を開けた

拍子に or 弾みに

The two are mostly interchangeable, and both are at home in conversation as well as writing. はずみ弾み is "bounce, momentum", so 弾みに suits a physical jolt – a fall, a collision, a sudden stop – and also appears as はずみ弾みで. ひょうし拍子 is "beat, timing" and puts the weight on the instant.

拍子に or たとたん

たとたん is only "the moment": the second event follows straight away, whether or not the first caused it. 拍子に says the first movement set off the second.

×外に出た拍子に、雨が降り出した is wrong: going outside did not make it rain.

After a noun, 拍子 is mostly met in the set phrase なんか何かのひょうし拍子に, "for some reason", "somehow".