Sound 'sounds' differently if it is travelling towards you or away from you. The most commonly used example of this is an ambulance siren which will change pitch at it flies past you. It sounds higher in pitch at it dashes down the round towards you and then slides into a deeper pitch at it passes and sets off into the distance again.
This is called the Doppler Effect and, simply put, is an explaination for how wavelengths change if relative speed changes. It even happens for light. Things appear more red if they move away from you for instance. This is something you won't tend to see in normal life as things have to travelled very fast.
So what's this got to do with film making? Well first off its interesting in itself and that should be enough! But of course, as ever, I do have a film making analogy up my sleeve.
Replace siren with idea.
A new idea can be distorted if it comes at you quickly. The speed of its approach to your mind distorts it and makes it sound better (or worse) than it might actually be. That flash of inspiration on the film set for a new shot, the sudden burst of ideas that means merging two characters in your script. Let them sit for a while first. Only an idea that has sat for a while and is stationary can be properly judged whether it is pitch perfect of not.
Next time you get that flash of inspiration - enjoy it - then remember the doppler effect.
If you fancy twatting about for a bit then explore waves with this java applet.