They fixed this bug in SWF 7, which to me seems like an awful lot of versions to not notice or fix that all of your bitmap blits are off by one pixel in each dimension. I guess Flash was primarily a vector animation format, not a pixel-perfect video game format. Still.
The reason I was getting bit by it despite the fact that SWFC totally supports up to like SWF version 9 is that it for some reason defaults to emitting SWF 6.