They didn't get a court summons but the court did call and send the plaintiff's filing. They were clearly not in the wrong in that case but it was still a hassle and quite a confusion. The point is people can sue you even if it's BS and you still need to respond.

In this case leaving a paper trail of having accessed unauthorized confidential information looks a lot like being in the wrong, so the potential hassle is a lot higher. You can argue it's not unauthorized after all, and you'll likely win, but you may need to expand time and energy arguing in the first place. And it could be significant.

Edit: In addition, (a) accidentally opening a confidential document -> oops, close immediately; and (b) taking a screenshot could be different legally (NAL yada yada), doing the latter could make it a lot harder to defend yourself.

Determining who is at fault involves extreme annoyance and inconvenience for those who had fingers pointed at them, regardless of whether or not they were actually involved. If you involve yourself willingly, you're inviting that on yourself.