Forgot to add that I tried dumping pacman's output to see if there's special unprintable characters that cause this and found no unprintables with vim and less.

edit - Some discussion on #bash on freenode has revealed that it is the environment exported to pacman due to `set -a' that's causing the issue, since pacman tries to import the environment twice and because `extglob' isn't exported, it fails the import. Making sure `set +a' is used before calling pacman should be a practical workaround. Thanks to twkm and geirha on #bash. Not sure if this counts as a pacman bug anymore, however, it does show that pacman spawns at least 2 new bash environments to do its printing, while it doesn't spawn any if printing is redirected. I guess it counts as a bug if this forking can be avoided, but I guess that's asking too much.

I don't see how there's anything here that could be considered a pacman bug. Pacman is a C program, it doesn't spawn anything for printing.