I saw one actually happen today;
101011 1346 3 -22 -0.3 18.106095 VB7JYV ME80 30 0 8668 -30

At 1346z there were three spots showing.
The top one was G4YND on 124 at -12dBm
The middle one was not decoded on approx 111 and very strong, possibly +3dBm
The bottom one was VB7JYV on 95 at -22dBm and thicker.

Messing about as though it was a mixing by-product....
124 - 111 = 13
111 - 13 = 98 which is within measurement tolerance of the middle spot

Earlier on at 1344z I also saw another one on approx 170 which didn't decode and was possibly from two strong spots on 210 and 190.
210 - 190 = 20
190 - 20 = 170

Just a theory - I've never been watching before when one happened!
I need to stay in more...

I've attached a screen shot (I hope).
And yes I know I need to do a freq calibration thank you!

73 Steve (A45SWL - awaiting license)