The Luxembourg Effect. Ever heard it?

I think that I've heard this in the 80's and 90's, but blew it off to IMD, [wshu.org]

Now, the effect that I want to know about, and others, is how is it at times on the 15M HAM band that you can hear your own signal 20min later come back to your receiver? Hmmmmmmmmmmmmm??? Art Bell even talked about that one happening to him.