这是一个不幸的错误。

It was an ___unfortunate mistake.

她丢了工作，而丈夫又病了，这真是太不幸了。

It was so ___unfortunate that she lost her job just as her husband got sick.

遗憾的是，他没能通知你关于会议的事。

It is ___unfortunate that he couldn't inform you about the conference.