Tonight I got a call from Shane Gaalaas, @Gaalbladderass the Drummer for the B'z, @Bz_Official who was mixing a song that had to be sent over to Japan tonight.  At a particular spot in the song, he would get a DAE error -6042 and playback would stop.  Here is what AVID has to say about it:

DAE error -6042 PCI bus too busy during playback or recording on a Macintosh computer.

If you repeatedly encounter -6042 errors, open the System Usage window and check the PCI gauge. If the gauge is peaked, you will need to reset the PCI bus. To reset the PCI bus, make all tracks inactive and start playback. If you still encounter a -6042 error, quit and relaunch Pro Tools, open the Playback Engine dialog, and change the Number of Voices to a setting that uses fewer voices per DSP.

Shane has a mac pro with a Magama expansion chassis running a 5 card AVID HD 10 system.  But even with all that power, running a large session at 96K was causing something to go wrong. I had him open the System Usage window and watch the PCI gauge to see what it said.  Most of the song it was around 35%, but at the point where the song stopped playing, it jumped to 100% and he got the DAE error.  Since Shane has 5 cards in his system, he had assigned most of his plugins to TDM instead of RTAS.  While looking at his System Usage window again, he told me that his CPU usage was under 50% and he was using all of his voices except 16 of them.  Well since the PCI bus was overloading, and all his plugins were loaded there, I had him take all of the plugins in his vocal chain and change them to RTAS from TDM. That decreased the amount of data on the PCI bus, as these RTAS plugins run on the CPU instead of the cards in the expansion Chassis. Although moving the plugins increased the CPU usage and the voice count, it decreased the PCI usage enough that he no longer got the DAE -6042 error and could complete his mix in time for his dead line of tonight.

Lesson learned tonight: To avoid errors during mixing, don't max out the system.  Make sure that none of you usage gauges are at maximum including the PCI buss, The CPU / RTAS, and the voice count.