>The question isn't whether or not it's a bug, but where it's coming from. VFP? or FPU?

This bug is documented in KB # Q157954 - "BUG: ROUND() Function Not Rounding Properly After Calculations". Unfortunately the article doesn't divulge exactly what conditions cause it to happen, it just says that "For some numeric values, the precision limit within FoxPro decreases after performing mathematical calculations."

I think you may be on to something when you found that the difference starts to show up at 12 digits (in your TRANSFORM example). The value of SET DECIMALS has an effect which also kicks in at 12 - see my reply to David Stevenson elsewhere in this thread.