Some time ago some USA WSPR TX stations moved from 503.9kHz on 501.1kHz to 
avoid RX blocking of local WSPR RX station., This allowed both USA stations 
(listening on 503.9) and  European stations (listening on 501.1) to copy WSPR 
DX without RX blocking by local TX stations .
But I noticed that in the following time more and more USA WSPR RX stations 
moved down to 501.1kHz too. And as a result some (at least one) European WSPR 
TX station moved down to 501.1kHz too (to be copied by the USA WSPR RX stations 
that moved down) and is now blocking the Europeans listening here.
So it seems that the RX blocking problem is back :-(
I guess this was not the goal ...

73, Rik  ON7YD - OR7T