I almost made myself completely deaf earlier by opening a recent project. My tinnitus is bad enough without the massive screeching feedback I was greeted with.
 
I opened a project I've been working on for the last 2 months - Input echo was enabled on 3 audio tracks. The inputs were set to my RME fireface mic inputs but no mics attached, and the mic input faders set to zero in RME Totalmix. 
 
This never used to happen unless a mic was actually hooked up.
 
What's changed? I just updated tonight to the latest version of Platinum.
 
 

This is happening with every project I open now...if my audio interface volume is up I'm getting massive feedback with input echo enabled audio tracks. I don't remember enabling these but they've never behaved like this before

I'm such a DONKEY! "Loopback" was still enabled in my RME interface. 
 
Ooops!
 

SONAR is pretty much incapable of feedback internally. I would guess something was looped back in TotalMix.