i have 16gb of Ram...though i must admit to loving the effects in Sonar Professional, i exercise a fair amount of restraint using mix busses and the like.
 
here's what's happening now:
i took a snapshot of my mix, and my master fader channel hit full peak in silence.
Right now, in this session i have nothing going above -6 db in the independent tracks.
i pressed e to disable the effects, and all played just fine.
pressed e again and this time i see activity in my track vu's but no sound, and peaks galore on my master vu.
gonna reboot and see if it helps.
meanwhile, has anyone seen this lil bugger before?

Update: reopened the session without restarting, and all is well.
just a little bug was all. 

Hmm. Glad it is resolved.

I see this problem almost every mixing session. It happens randomly - if I could find a reproducible recipe I'd log it as a bug. For me, it seems to be related to using Ozone on the master bus and/or the Sonitus reverb on an aux bus, but can be triggered by almost anything, even just hitting play. Disabling FX stops it. I have managed to write out a wav file while this is happening, and all the values are -32767 (the maximum negative 16 bit integer).
 
Let us know if it happens again, and particularly if you can find a repeatable way to make it happen. There is a gremlin lurking (others have seen it) and it would be good to get it nailed.
 
Cheers, Peter.

The most common cause used to be Concrete Limiter, but those days are behind us I think.