accidentally-on-purpose：故意装的出于偶然地，似为偶然实则有意地

e.g. Then I accidentally-on-purpose spilled water on him.

那会儿我是存心要把水泼在他身上的，只是装出一幅碰巧不巧的样子罢了。

e.g. I knew it was done accidentally-on-purpose. That kind of thing doesn't just happen.

我知道这件事貌似出于偶然实际上是故意的。那档子事哪会说来就来呢！