In a safe place.

If you have something you don't want to lose, you might put it in a safe place. Sometimes, however, you put it in such a 'safe place' that later you can't find it.

Example:
-Do you have the tickets for tonight?
-I put them in a safe place and now I can't find them.

Good luck with that!

'Good luck!' can mean just what it says -- that the person is wishing you well in your endeavor. However, 'Good luck with that,' especially when said with a sarcastic tone, means that the other person doesn't think you're going to be successful at all.

Example:
-I'm gonna go cram for my Philosophy final tomorrow.
-Yeah, good luck with that!