My friends and I bought 20S units for our annual 10 day trip.  We upgraded to the latest and greatest firmware (1.4.1)  Two of us have date codes from 2015 and the third has from 1408.  So we all have new enough hardware that we shouldn't have any problems.  We found on occasion when intercomed we would get this unbelievable feedback in our helmets.  We eventually figured out what was happening and it is a very reproducible problem and a very annoying one when you have guys that can't remember who they are intercomed to.  So I'd like to start off by saying the group intercom feature is very buggy and worked horrible for us.  It looked up regularly and when it did lock up, you generally couldn't intercom anyone anymore until you rebooted your 20s .  They would have to reboot at the next stop when I asked them to reboot.  So by the end of the first day we all agreed that we had enough of that feature (that's a loose term in my opinion).  So we decided to intercom directly to each other.  And since we only had 2 other guys to intercom to, it wasn't that much of a hassle to connect to rider 1 and rider 2 shortly after.  So here's how we can make the feedback happen.  I'm rider 1 and I connect to rider 2 and rider 3.  I'm the common source for the group intercom.  But then rider 2 wants to talk to rider 3 and forgets that he's already connected via rider 1.  So rider 2 tries his own connection and it is successfully created between them.  But the problem is we are already all connected via rider 1.  So the feedback starts right away and its LOUD.  He almost crashed his bike the first time it happened.  So the Sena is not smart enough to figure out that it has a connection to another 20S, even though is via another 20S.  This is a huge flaw and I can't believe nobody at Sena has not tested this.  This needs to get fixed as its a bug and happened to us on a regular basis on the ride.  In fact, it happened many times by accident as rider 2 would try to start his music and didn't hold the button long enough so the 20s saw the press as a intercom command.  If you spend 10 days with these units, you'd see how easy it is to have this happen.  I'd really like to see Sena start to fix the existing products they have and stop pushing out all these new ones.