Every car ive ever driven has done this at one stage or another... Not every time i put the gear in reverse, but only occasionally- it makes some interesting crunching gears. I dont think it's serious at all but I was wondering if anyone knows why it happens?

There are two main reasons for crunching into reverse.

1. You have not allowed the rotating gears to come to rest after depressing the clutch pedal. If the box was in neutral, the input shaft & cluster gear are rotating. Depress clutch fully & wait a second or two to allow gears to stop..... Select reverse.

2. The clutch does not fully release & the resultant drag on the clutch disc prevents the gears from coming to a stop. This can be caused by
A. Lazy leg syndrome [push harder]
B. Clutch badly adjusted. [adjust it]
C. Oil on clutch from engine or gearbox [Pull box out & rectify cause, then replace disc, or whole clutch]
D. Unservicable or faulty clutch. [Replace it]

Reverse is the only gear without synchro, & two gears actually slide together, unkike the foward gears which use a diferent system alltogether. One of these gears will be stationary if the car is stationary, so if you try to engage a rotating gear with a stationary one,... well you know.
Bring the rotating gears to a stop & they can not crunch. If you like, you can just "lean" on one of the foward gear positions to use its synchro as a brake for the rotating parts, then select reverse.
Try not to select reverse while traveling foward, it's bad for the nerves, the box, & your wallet.

just when you are actually putting it into gear?

as reverse doesnt have a syncro on it, usually if you are revving stightly higher than idle it will do it, not really a concern

[edit] posted same time as dogemans, his is a little bit more technical than mine

Thanks guys, i was always really curious about that. The fact that it only ever happened very occasionally made me think it was probably something i was doing wrong.

Now i know it was probably me doing something wrong

Right,...but we have ALL done it.

Astro boy.

It is nothing to worry about.
My 1200GX also did it.
Especially on very hot days.
My dad told me a trick that I use on all cars that I drive and it has always worked.
Hot or cold weather.
Put the car in second gear and then select reverse when you want to reverse.
It never failed me.
I do it with my datsun.
Can select reverse quickly with no problem.
Never select reverse when your car is moving forward or backward.Even when moving very slowly.