I kept hearing this the other evening in a TV drama.

Problem:
“Safety” is the wrong adjectival modifier here.

Explanation:
I was watching an episode of the Fox TV show “Fringe”, and the drama began with thieves using a bizarre method to break into a bank safe filled with depositors’ boxes so that they could steal a particular box.

All of the actors referred to the mystery box in question as a “safety deposit box”.

The correct phrase is “safe deposit box” because the deposit boxes reside in a safe.

Using “safety” instead of “safe” in front of “deposit box” seems to be a common English blunder.

For fun, I searched Google for each of the following (with the quotation marks, to avoid variations) and got about the indicated numbers of matches:

• “safe deposit box” — 4,570,000 matches
• “safety deposit box” — 2,000,000 matches

This tells me that Web authors have used the correct “safe deposit box” versus the incorrect “safe deposit box” by a ratio of 2.29-to-1, which is awful.

Solution:
“Safe Deposit Box”