Stapleford has a shared frequency with Nottingham, and sometimes we can pick up their airborne radio calls. It seems to depend very much on the atmospheric conditions, although altitude must also be a factor.
I've also picked up other radio traffic inbound to Bourn. The first time it happened it was quite confusing as they were referring to runway directions which were completely different from what I was expecting and I though I had the wrong frequency.
The most bizarre occurrence was when I was flying to Earls Colne in a club C152 which didn't have the .25 steps on the frequencies. Having tried 122.4 without success I switched to 122.45 overhead Witham and got a reply from Goodwood. It's their frequency, but it was a good 75nm away as the crow flies, so it shows how far away you can pick up interference in the right conditions.
I assume (never assume anything!) the ground based station doesn't pick up the same interference, so if you happen to talk across a transmission it doesn't matter. But it is rather off-putting if you're trying to make a circuit call and can't work out whether the chatter is local traffic or not.
It's just one of the things that makes flying 'interesting'.
RD