On my first day in broadcasting I was introduced to the reel to reel tape machine - a Lyrec from memory. I was then introduced to the razor blade and how to edit audio using the two. Next piece of information was that razor blade did not live on the tape machine. To explain why I was shown how quick the rewind and fast forward spun the tape and reels. "Imagine that going at speed and a razor blade going with it". I agreed and was actually very nervous I'd do it. Then I thought that the razor blade was quite light. It would therefore be unlikely to stay on the reel whilst it got up to speed.
It was only later in the day that I spotted the some marks in the wall at desk height. Looking at the size and shape they were obviously from razor blades that had been left on the machine. Someone told me subsequently that if you left your blade towards the centre of the reel it could get up to some speed before coming off. I bought a magnet that sat in my desk drawer and I covered one end with some loosely held hard plastic. This held the razor blade securely covering the sharp edge when I wasn't using it.