I was going to mention it, but it slipped my mind.

我正要说到它，可是想不起来了。

Sorry, it slipped my mind.

对不起，我不小心给忘了。

I was just so busy it slipped my mind.

但是我太忙了，所以忘记了。

It slipped my mind.

从大脑里溜走了。

I did mean to call her, but it slipped my mind.

我确实想打电话给她，但不留神我忘记了。