I haven't fully investigated this, but doing an update on trunk today I started receiving the following error message:
This is occurring when a row is updated to point to the related field in another row (when both are persisted at the same time). It doesn't actually break the updates (with xdebug installed anyway).
It only appears to occur with a select few of my models that contain cascaded persists (and I'll be sure to attempt a test case for this for tomorrow). I haven't tested this fully but wanted to post this in case anyone has an idea of why this might be happening.