Can't copy the same publication to two different series' in the same cycle

[Adam Conrad]

Copying to two pockets in the same series in the same publisher cycle works fine (ie: to -security and -updates), but copying to two series' at the same time ends up failing with a message like so (this is a reject from copying to raring after copying to quantal in the same cycle):

Rejected:
foo 1.2.3-1 in quantal (same version has unpublished binaries in the destination archive for Quantal, please wait for them to be published before copying)

I can't quite see why this restriction would exist.