I managed to copy the same package from one PPA to another, twice. This may have caused https://wiki.canonical.com/IncidentReports/2009-04-06-PPA-Publishing , so marking this a security vulnerability.
Here's what I did (memory is a little fuzzy on details like error messages):
Using the +copy-packages page in LP, I copied a package from a source PPA to a destination PPA. I thought I chose the 'Copy existing binaries' radio button, but the result of the copy according to the message from LP was something to the effect that only the source package was copied to the new PPA (it listed just the .dsc and .tar.gz as being copied).
At this point I believe I checked the destination PPA's page in LP and saw that it was empty (no packages) though I'm entirely sure on this.
Next I re-attempted the copy, this time making sure that the 'Copy existing binaries' option was selected. This copy operation appeared to succeed - LP reported that both source packages and binaries were copied to the destination PPA.
A few minutes later I confirmed that source package + binaries were present in the destination PPA.
Package: canonical-oem-keyring - 2009.07.23+build2
Source PPA: https://edge.launchpad.net/~oem-archive/+archive/trainingkylen/
Dest PPA: https://edge.launchpad.net/~oem-archive/+archive/trainingawolfson/
This indeed resulted in publishing for PPAs to screech to a halt. After deleting canonical- oem-keyring from the destination archive, canonical- oem-keyring was once again published to the archive and normal publishing for all PPAs resumed. I'll update the incident report with the technical details.