1.   I meant to buy some milk, but it completely slipped my mind.

2.   It had slipped his mind entirely that today.

3.   It had temporarily slipped my mind, but some one did have a spare key some while ago.

4.   It slipped my mind because of the tragedy that followed.

5.   Yes, that had slipped her mind.

6.   You think something as important as that would slip my mind?

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

8.   The reason for my visit had obviously slipped his mind.

9.   Maybe it had slipped my mind.

10.   How could something as important as that have slipped her mind?