Mixdown - vocals go out of time?

Here's a weird one. I have recorded a tune with 6 instrument tracks and 2 vocal tracks. It all sounds fine in the multitrack and all the clips are all locked in time.

When I mixdown and listen to the mixdown both the vocal tracks are out with the instruments and with each other. It's not subtle either. They are nearly half a chorus out!!

I've not had this with any other recording from the same session. Anyone know what's going on?

............and just to prove me a liar. It's all OK again after I saved the session, shut AA2 down started up again and re-opened the session  .

I'm still interested to know, has this happened to anyone before?

It's never happened to me, but there have been other stories about AA2.0 tracks jumping anything up to half a second before - and nobody seems to know why this is, I'm afraid.

Mmmmmmmm

Happened to me a few times today with other tracks. The workaround seems to be to save session, close all, then re-open session. This seems to re-sync everything (and re-initialise the effects rack, but that's another story...) just before doing a mixdown.

Yes, and the effects rack has been implicated as well previously...