One time I also opened up a 工資單 that didn't belong to me.

In my case, the associate medical director (he was doing the hand-out of the 工資單 for the medical director who was on vacation) handed me the 工資單 in my office, and I opened it up immediatley without looking the name on the envelope, then I realized that that 工資單 was not mine, and gave back to the associate director right away.