Chat supports oneboxing for Launchpad bugs for projects other than Ubuntu. For example, linking to https://bugs.launchpad.net/inkscape/+bug/889421 shows the bug description.
However, bugs in Ubuntu use a different URL scheme, which also specifies the package name. I tried linking to Launchpad bug #612986,
https://bugs.launchpad.net/ubuntu/+source/fortune-mod/+bug/612986, in chat. It does not onebox. Currently, to get it to work, you have to manually edit the link to
This nerfs the oneboxing, since most Ubuntu bugs redirect to a URL containing the package.
The fix is to also recognize Launchpad URLs containing