One more point on the Mayor who promised to build the ‘spamusement’ onsen amusement park if their video hit 1 million views— the phrase that got picked up by the news is a handy pattern that shows up pretty often: “yatte shimatta” (top right of image)
For te-shimatta, I always think of someone who simply couldn’t help themselves and went and….. (insert verb here). So, if you think of someone who meant to bring a box of donuts into the office, but got stuck in traffic and ended up eating the whole box. Japanese could say (食べて)+しまった
You can do the same thing with virtually any verb (飲んでしまった、等）but you can cover many situations by using やってしまった。
The beauty of the internet being what it is, there is really no shortage of examples of things online– not just using Google News (searching for やってしまった）, as I suggested the other day, but you can easily find comments and suggestions from other folks who are learning Japanese. For example, I found a page on Quora on this topic: https://www.quora.com/What-does-shimatta-mean-in-Japanese-Does-it-have-more-than-one-meaning.
I couldn’t find a link to someone using Pandora’s Box as an example for this pattern (ie 開けてしまった or 見てしまいました）, but I think it would be another good example of this pattern.
* locksleyu wisely left a comment (see below) to point out that this pattern ( ってしまう) is bigger than the example I’ve raised. Indeed, this pattern can capture a few different nuances! The challenge for English speakers is that none of the English translations will ever roll off our tongue quite as smoothly as the Japanese.
Thinking of another example, I remember I was once moved to tears (happy tears) after the one and only time I had ever flown in a helicopter—a Japanese colleague, seeing that I was embarrassed, used this form to note the moment: “感動しちゃった”.